Go to Options > Game Settings and use these settings:
Difficulty: 1 Star
Damage Level: 8 Stars
Max Rounds: 1
On the lowest difficulty, the AI hardly ever blocks or attacks. With max damage, everything dies in a few hits. Most of the time, you can just jump towards your opponent with and over and over.
Juggernaut is especially effective because he has super armor. This means he can absorb 1 hit before entering hit stun. His + move makes him travel all the way across the screen and does huge damage. It only takes 3 or 4 of these to kill a character. This combined with super armor lets you plow through everything easily. If you've used Zangief's Lariat in Street Fighter 4, or Sentinel's cr.M in MVC3, Juggernaut's + is similar to that.
Thanos will be the hardest part, but the only real difference is that you won't be able to use gems for this fight. Just keep the pressure on him and avoid his big attacks.
If you're going for "Marvel Legend" you can hit on a second controller to start a local match and save yourself whenever you think you're about to lose. Otherwise, you can use as many continues as you want. You can also switch characters whenever you used a continue. If there's a character your comfortable with, you can use them up to Thanos, lose, use a continue, and switch to the characters you need to beat the game with.
You can also see a complete playthrough by RA1NKING HERE.