Anyone have any advice? STE, maybe?
This is a horrid matchup… I have my own ideas of what might work, but taking advice from those who’ve done the matchup more would be helpful.
P.S. I know, MM shouldn’t fight Sent at all… even I only use him as a countercharacter, but the match inevitably comes up and I want SOMETHING I can do, other than “DHC in Storm” 
Allow me. 
While granted I will give it to you that this is difficult and I hate this matchup, I can give you a few hints:
If you’re playing a ground-trap Sentinel or playing against a Storm assist, try to bait the trap out and go overhead. This will usually bring a wake-up call, as most people don’t expect it. Good players will counter by using AAA’s, so watch it. If you have Super, try the Beat Plane.
Flying Sentinel: Make sure your assist is good. CapCom and Ken are the best at stopping this: CapCom takes out all air and Ken goes through their pesky assists. You can also use an expansionist tactic with CapCom also to wipe the floor with Sentinel. I don’t particlarly like Cammy because she hits the assist and not Sentinel. If the Sentinel player is good they’ll bait your assist, so be careful.
Beamer (FP) Sentinel: Break players will kill me for this, but jump back and time your own Jumping Fierce Punch to hit them, then get ready to block. Regular Jump half-way across with an assist as an expansionist tactic also works well. Just don’t do it full-screen, or your assist will eat Hyper Sentinel Force, and you’re in a huge hole.
There’s a start. Try these out and hopefully you will improve.
-NJS-
Megaman vs Sentinel is not really his worst matchup, I think he has more trouble against Cable/AAA than anything else.
In his matchup with Sent, you have to make sure you know how to use his projectiles well(not his fierce). By this I mean, you have to adjust which one you want to use either Rockball or Tornado Hold.
Most Sents play with either Commando, Cyclops, or Psylocke AAA’s. In those case you can either counter with Tron assist, Commando, or Sent ground. In any of these situations, we’re gonna assume you both can cancel out each other assists bringing it down to the one-on-one fight.
When fighting Sent, be sure to remember that Megaman’s jumping fierce cancels out most of Sents regular attacks when he’s ranged. If he’s upclose he’ll trade, so you need to watch the spacing when you are throwing fierces.
Rockball is EXTREMELY effective against a flying Sent. It disrupts his patterns and combos. And once in a while if you can catch him, SJ jab, short, Hyper Megaman DHC into whatever if you want, or simply do a small SJ jab, short, fierce for a nice chunk of damage.
Another thing is if you SJ over his HSF, you could always use Beat Plane.
The matchup here is about the same with all his fights. You just need more patience in this fight so you minimize the damage you take.
That is if you have time to get his Rock Ball even out: Sentinel can knock you into all types of traps in a very fast matter (Sentinel Force/Clones, LP/Hyper Sentinel Force)
I find the Rock Ball is mostly useless because it’s impractical to get it out anytime fast. Magneto rushes you down, Storm with hailstorm, Sentinel as described, and Cable shoots you.
-NJS-
Sentinel trapping megaman… its sorta a blessing in disguise.
When ur trapped against a keepaway sentinel, that’s really your best time to charge your blast. hyper sentinel force is a pretty good time to shoot a well charged blast. Each wave is three hits, and a well charged hp, due to its size, will negate 2 of the the 3 hits that a wave HSF would do. Megaman ducking will keep him safe, even if the HSF survives the first wave only.
Fly sentinel is susceptable to uncharged air hp and rockball and pretty much ANY anti air assist. Firing 2 shots in the air covers the assist nicely.
i dont know too much with tornado hold, but alot of people run into it. My strategy is a bit old, as i havent touched this game in a few months.
Rockball is very useful its actually not that hard to get out. Against Sent, you only need it if he’s flying around, so once he super jumps, call it out. You don’t have to kick it right away, so you can wait and kick it whenever. It depends on what kinda team you’re playing vs what they’re playing. Matchups, etc. Most successful Megaman teams I play with involve either MM/Sent, or MM/Tron. In either case, the traps often trade. If Sent stays on the ground trying to trap you, just use tornado hold. It cancel all of the drones and hits sent if he tries to low fierce. You can still rush a grounded Sent, I usually do with Tron assist backing though.
Brief description of what I do against the characters you described:
Magneto: I don’t even care what projectile I have, I’m just using jump fierce in this matchup. Most times Magneto is paired with Psylocke, Cyclops, and sometimes Commando or Sent. In either of these scenarios, Tron or Sent assist either beats or trades with his assists. Mag/Psy is usually pretty scary, but with Tron assist, it trades with Psy most of the time and hits a big chunk of damage on that assist. Leaving Mags vs MM. I like to mix up jumping fierces up with jump short, short, fierce. MM’s jumping short is pretty good, watch for the pattern of Mags jumps, you can see when he’s diving, time the fierces. Try to be random and watch what’s going on and you’ll eventually force Mags to not rush constantly.
Storm: Oh man, rockball is SO good against her. You have to get it off early though. Of course if you do it at the same time Storm does typhoonXXhailstorm, you’re hitting whacked. But if you do it early, it knocks her out of it. It’s even better against runaway Storm. I find rockball to be one of the most effective ways to catching runaway Storm. Granted its not 100% perfect, but nothing really is when Storm puts on the track shoes. Randomizing jump fierces is good, and that mid-range jump fierce. If you play Megaman, its pretty much the same spot as it was in MvC1, the spot that always hits the medium sized character in the head. It goes over typhoon and hits Storm out of hailstorm.
Cable: This is just flat out patience. You can still rockball, I utilize the slide to kick it though in this matchup instead of low short. Tornado hold is better in this matchup. Cable is the slowest of the top 4, so you can sorta rush in on him with jump shorts. I try not to fierce as much in this one, cause Ghostbuster gun>>>Megabuster. :lame: I hate Cable, I hate playing against him, I hate playing him. But I have to say that this match up to me is the most fun because it becomes a chess game. How can I hit him with fierces without gettin shot.
NJames: I’m not the best MM player out there, I’ve read some of your stuff in the other Megaman thread, so I know you know what you are talking about. This is just some of my experience playing in tournaments and around here. I’ve been playing Megaman hardcore since MvC1, he’s a fun little bugger. Were you at EVO? I still don’t see too many MM players, especially not the hardcore tournament ones, we need to stick together 
I’m an East-Coaster, was there for B5, lost to Rio, but not at EVO.
I’m more known as a FP Mega Buster abuser, but I know how to use it. I’m also known to sucker Mags and rushdown Storms into assists (especially Ken’s).
I’ve been thinking about a new trap with Mega Man, but I got Sentinel-trapped the other day. It basically entails:
Rockball --> Sentinel Assist --> Jump Back FP --> Ground FP --> Repeat
If anyone can pull this off let me know.
-NJS-
That’s pretty much the most effective Megaman trap. It starts with the drones flying, jump back fierce, ground fierce, kick rockball, call drones, jump fierce, land fierce, rockball, etc. At least thats how I do it. It’s gets owned by commando though. I just do for damage, which is why I like Tron assist. If you play against Mags or Storm, or any other rush down style, Tron is definitely better, especially if you bait them into Ken. Baiting into Tron is much more satisfying.
I’ll have to try that particular one, I might be able to hit my triple-delay combo off of a Tron assist (almost insta-kills characters)
Yeah, expansionist Commando is a major pain in the ass, that’s sometimes why I prefer Ken assist (goes right through that)
-NJS-
Only a few assist really rock Tron assist for free, one is Ken, but he’s hardly used on serious tournament teams nowadays. Against Commando AAA, Tron beats it most of the time. If they’re called out at the same time, she hits him out of corridor. If she a little early, she still beats him. And you could always use her as a counter assist and kill em off.
A few things I like to do with Megaman/Tron:
SJ jab,short, Hyper Megaman is pretty much worthless nowadays, you get almost the same damage from his regular air combo with the fierce. Its only real good for DHC’ing Megaman out.
Call Tron, air throw, jump fierce, s. roundhouse, sj jab, short, jab, short, fierce. Nice damage, if they tech, throw immediate air fierce, then j. jab, short, jab, short, fierce.
Guardbreak mixup. Guard break with jump fierce then do one of these three things:
a. Call Tron, jump throw, s. roundhouse, air combo
b. Call Tron, low short, short, fierce, stand short, roundhouse, air combo
c. Call Tron, SJ, on the way down short, fierce, s. roundhouse, air combo
Giving me some food for thought, I think I might actually try out this stuff. (hehe)
As long as I don’t have to play Magneto. 
-NJS-
The posts here are really good, but not a lot of them have to do with MM vs. Sentinel.
Keep on topic, please. I’m not trying to be an arse, I just want to make sure this latest change in SRK is successful.
I found some things that have greatly helped my anti-Sentinel game, and my Megaman does MUCH better on Sentinels now.
Something people never remember about Sentinel - is that he can only fly as high as the screen allows him to. This means he cannot take to the superjump space if you don’t jump. This also means that jumpback fierce is actually a potent weapon on flying Sentinel. If you can fire a fierce high enough, he’ll be able to fly over it, but as you land from the jump he will actually fall on top of it. Tack that on with the drones to prevent Sentinel from actually attacking you from a fly, and you get to use jumping fierce again. Couple that with Rockball, and Sentinel’s flying game is GONE. Tight!
Now for ground game. Sentinel still has his full screen poke, but Megaman is not without options. Trapping MM with fierce into drones is almost impossible and not worth trying (slide your way over to him if he does). Also, blocking an uncovered low fierce w/o HSF means you get to put drones and a low fierce of your own on the screen for free. If you’re made to block for extended periods of time on the ground (NOT in the air) you can charge your buster and fire it when you stop blocking. Charged busters break super armor, so Sentinel can’t plow through that shit. Now you actually have something that can take Sentinel on with.
You’re my hero:(
Everything I do against Sent is covered here mostly. If he’s in the air FPs while forcing him into it. A mistake I’ve made is to try to punish flying sent with Beat Plane, NEVER do this, he flys faster then the bullets. Against grounded sent, just the opposite, I try to chip with Beat Plane all the time, and I dunno how but sometimes the heavy SOB falls out of the launch, SJ.LP,SJ.LK XX HMM combo. This as with Cable is a match where you charge(unless he’s flying) the buster, as perviously stated it will beat out HSF, also as stated if he wiffs HSF I attmept to BP but I have been hit by drones and comboed into so its risky. I try to avoid being trapped with random slides and wall jumps. The RB trap is useless against speedy armored opponents, Collossus, Silver Samurai, Juggs(Only cause of Head Crush), and Sent will eat the RB trap…luckly Sent is the only Top Tier where as the learning curve for the other char’s is too high where their actually a threat. I can’t say much else, I think this is a worser match then Cable…I know you said you didnt wanna hear this but you got a better chance to beat Sent with your other characters. I don’t see a reason to put MM up on him. if I ever get stuck 1 on 1 then I gotta realize I fucked up by not taking him out eariler. It isn’t only about knowing how to use him, its knowing when to use him…its like putting Cable on Mags 1 on 1… the few things you can do will probably be in vain but good luck.