Have you used SRK’s MvC3 guide? It’s actually pretty solid through and through. If you wanna step up from there, get the BradyGames guide… which is gold. There’s a /ton/ of information there, and it’s totally worth reading.
If you’re having trouble landing hits, then one of two things are happening: You’re getting hit too often to attack or he’s blocking all of your attacks. To both of these things, the solutions seem fairly obvious, but you have to approach things analytically to see them.
First, if you’re getting hit too much, learn to block better. There is more to blocking that simply holding back. You have to block in different places at different times, and learning to predict which kind of block you’ll need when is rule number one in defense. If you want to level up your blocking, one option is to open training mode, turn the enemy AI on to a high level, and never attack. Don’t get frustrated when the AI breaks through your defense; it’ll happen. Just focus on building up your blocking reflexes. With infinite life, you can do this for as long as your attention span allows, and I’m sure it will help you in the long run.
If, on the other hand, he’s outblocking all of your attacks, it’s most likely time to learn offensive variety. It’s absolutely essential to learn how to mix up your attacks. Never, ever spam the same special attack or combo attempt when it’s failing most of the time. Instead (refer to either guide as necessary for more information on how), you have to blend lows, mids, overheads, crossups, traps, throws, projectiles, etc. You play Deadpool, and he has a great number of options (so do Dante and Taskmaster for that matter). Refer to the character specific guides to find out which attacks hit where, so pick the fastest and safest ones (to give the least amount of time to block on reflex or punish with a counter) from each level (ie, low, mid, high) and use them, doing your best to look at your opponent before attacking. Hit him where it will be hardest to block. I’m pretty sure training mode has some form of random block option for the AI. If so, set it to that (instead of having it off or on CPU proper) and work on your attack variety.
Once you start to land individual hits, be sure to learn the Magic Series (which varies from character to character in exactly how many hits you can get out of it, but usually goes :l: :h:). There are some super easy combos that are practically universal, generally easy to pull off, and do alright damage. :l: :h: :s: :u: :h: :s: for example. The important thing to keep in mind about Magic Series is that if an attack connects (isn’t blocked), you can usually use the next most powerful attack in the series and get a guaranteed hit. Check around for full details, but, as a rule, you can cancel normal attacks into special attacks, so avoid landing just one hit. I don’t know for sure that it works, but I think, for Deadpool, :l: :h: :qcf:+:m: would work. You do more damage than if you just did :l: :h:… and a ton more damage than if you just did :h: or even :qcf:+:m:. Training mode’s auto block option is great for helping with combo practice on any level; the AI will try to block any successive hits after the first one (which it doesn’t block). If a combo works on auto block AI, it should work against a real opponent (barring execution errors, of course).
If I’m getting too fundamental, I apologize. Sounds like you want absolute foundation stuff, though, so that’s what I’m aiming for.
Going character specific for a moment, I love using Deadpool against Sentinel. When Sent is on point, super jump becomes your friend. You get out of a lot of Sent’s firing zone and Deadpool has at least a few options for hitting someone from that height. Big fan of his grenade (PINEAPPLE SURPRISE!), but his downward aiming Trigger Happy will often hit a ranged spamming Sent. If you’re feeling gutsy, take the fight straight to Sentinel by super jumping toward him, use one of those specials along the way, then go straight into your melee attacks once you’re close enough. You can find anti-Sent strategies all over.
Hope some of that helps.