1. Execute your combos - This is one of the biggest hurdles people seem to face even with Marvel's very easy combo system. Execution of your combos means you take advantage of your hits which may not come easily against someone well seasoned in the game. This tip also implies doing the right combos. Research your characters and find out what combos are best for your team structure and personal play style. This is where videos help the most I think.
2. Use assists effectively - To Marvel newbies, assists are a very foreign concept. But put simply, assists are the most valuable tools in the game. They can be used to cover, defend, combo, mixup, and more! Remember, assist cooldowns are very short. If all else fails, when you're first starting out- do as my good friend Jayford says- "Spam the broken assist."
3. The best defense is a good offense - Marvel has always been a game of push buttons now, ask questions later and the games have been designed with the premise that rush down is king. This game is certainly no different with characters that have many offensive tools but lack adequate defense against them. When I first played a good Wolverine, I asked the player "How do you deal with that?" All he could say was "Don't let me get started." So true. As long as you keep the pressure on yourself, you can never be pressured.
4. Tag out your injured character - Having a fully comprised team is so huge in this game that saving all the red life you can while helping a severely hurt character survive should be a top priority.
5. Tag in/out safely - Learn to not rely on raw tagging all the time. Safe DHCs (Delayed Hyper Combos) and TACs (Team Aerial Combos) are the safest ways of tagging in. Of course you can always tag when you have killed off a character.
6. Kill off characters - Do whatever it takes. Killing a character is huge and 9 out of 10 times it's the right decision. As you become more advanced, you will know when to kill and when to pursue other avenues.
7. Use TACs - Team Aerial Combos offer little risk and a bounty of reward to the player using them. Use and abuse these for big damage, free meter, and a safe tag.
8. Use Snap-backs - Snap-backs are even more useful now than in previous Versus games. They can be combo'd into from any standing H and have tons of uses. Snap in the character who is close to death so he can't get red life back. Snap in your opponents best character if you know it. Snap in the team anchor to kill the come back factor and mess up the team order. Snap in the problem assist like Tron or Haggar. Snap in Phoenix.
9. Proper X-Factor use - Never die without activating!
10. Practice. Practice. Practice.