Marvel vs. Capcom 3: Fate of Two Worlds

To speed up the time it takes to beat this mode I suggest putting the damage to Highest and the timer to 99. You are able to select these after selecting your team for the first time in arcade mode.

I found the easiest way to beat this mode is with my team of Iron Man (Proton Cannon), Hulk (Gamma Charge) and Magento (Magnetic Disruptor)/Another Beamer.

Using Iron Man as your first character, simply spam Unibeam (Quarter Circle + Att) while assisting with Hulk. This will lock down the CPU while doing a sizable amount of chip damage. When Iron Man gets too damaged or incapacitated, switch to your other beamer and repeat. If it really goes down to it, when your Hulk is out make sure to use your X Factor (RT + A) and you should be able to gamma charge them to death (Quarter Circle + Att).

When it comes to Galactis, the first part is easy since it follows the same pattern as the prior rounds. When he reaches his second form, try to jump up to his face and continually beam him (Do not use your assist).

If you do not kill him with your 2 Beamers, your X factored Hulk will be more than enough to kill him (Make sure to save your Special Bars). With all the combo bars, simply use Hulk's Gamma Crush (Back Quarter Circle + Att x2) and win.

The achievement will unlock after the credits (When it shows the final score).

Main with sentinel and have two allies with beam assists like iron man, magneto, and doctor doom. For every match use sentinel force with light attack. The computer will take heavy damage and will have a hard time avoiding it. After calling sentinel force you can cancel into another one very quickly. In-between call your assists for extra damage.

When it's time for galactus you can use the same strategy against his heralds. But save your hypers for fighting galactus.

The freakingly easiest way to achieve this is:
Team:
Sentinel(alpha),
Iron Man(alpha) and
Dr. Doom (alpha).

Set the rules for Very Hard (of course), 30 seconds, MAX damage.

Keep defending with Sentinel while you call Iron Man or Dr. Doom (alternate them Lb-Rb-Lb-Rb...).
When you have a Level 2 gauge for hyper combos, press LT for a Crossover combination if you want to finish quickly.

With Galactus minions, repeat the process; to beat Galactus punch him with your best moves; if Galactus starts to do his "Squishin'" combo and you have a decent Level 2 or 3 gauge, activate the Crossover Combination to nullify that move. Keep hitting and the big fella' must fell.

I know this is a spammer way but If you're really desperate, this could be the very best solution.

Go to Xbox Live mode and select Event Mode. Find the event that makes you play through Arcade Mode in 1v1 battles and select it, then go to Arcade mode. This is the absolute fastest way to get through it and you won't have to rely on assist spam.

Pick your best character, or better yet one of the stronger guys (Hulk or Sentinel for example). The cpu will try snapbacks on you, but since there's no partners/teams it won't work. Guys like Hulk can use this as an opening for a combo. If you activate X-Factor (absolute must) then you can kill them with a couple good combos. Be careful, because the cpu will abuse X-Factor also.

When you get to Galactus, save your X-Factor for him and don't use it against his heralds. All your attacks will do more damage and you'll build meter fast, so spam him with your character's most damaging hyper combo.

My solution is using Sentinel(doesn't matter about assist), Hulk (gamma charge), and another person who can shoot as an assist. I used Iron man for his unibeam assist. Put settings on very hard, damage highest, time limit if you really want it. I prefer not. So spam Sentinels beam with heavy attack, then crouch and heavy attack. Stay away from the opponent. When he gets close, use Hulk assist. It pushes him away. Then repeat. You may die with sentinel, if you do, substitute other shooter (Iron Man for me) and repeat with Unibeam. Continue that until boss battle. In boss battle attack the two metallish guys the same way. When it comes to Galactus, substitute in Hulk, use X factor, and quarter roll down to back and hit two attack buttons to use special. You should have 5 special bars, so use that special five times. Galactus should have minimal life left, and just finish him. You should get the achievement.

Note: I died three times during this strategy. You will/may die. If so, continue. You will not lose your achievement and you will lose all progress if you give up halfway through. Good luck.