My preferred team for unlocking this was Mega Man and War Machine as they are easy characters to cheese projectiles with. First and foremost, you'll want to go to the game settings under options and lower the difficulty (although in my opinion it doesn't help much) and raise the damage level to maximum. That second part may vary depending on your play style, however I preferred doing big damage to the boss and defeating him with some meter rather than being able to endure his attacks.
Moves you can use to cheese your way through the first few fights leading up to the boss are as follows...
Quarter Circle forward+ Triangle using War Machine gives you the shoulder cannon. This is an effective projectile that pushes them back and is fairly powerful. Megaman also has a move with the same input that throws down a mine which pops up the enemies. This can certainly be effective.
Using Megaman, I personally just liked spamming R1 presses as this is his mega buster move. He simply fires from his arm at a fairly rapid pace which can keep many enemies at bay for a fair amount of time. This can be done both on the ground and in the air. Often times when calling in Megaman, I would mash it in order to get one as soon as I came out and usually it kept the enemy from catching me in the air.
Next, we'll go in to how to utilize your meter to the fullest. Both of these characters have solid projectile hypers that can be used by pressing Quarter Circle Forward + Triangle and Square. Both of these can take off a huge chunk of damage if you land them, though the Ai occasionally blocks them. If you have 2 bars of meter, you can do a Quarter Circle Back + R1 and R2 to have both characters jump on to the screen and attack your foe. Since you have unlimited meter, usuing QCF + Square and Triangle repeatedly works well. I usually waited until the foe attacked my other character before using my first hyper, then the AI would follow my lead and use their own for huge damage.
A huge part of this process is patience. There are times when the AI will be unrelenting despite the fact you just turned down the difficulty. Characters like Gambit, Strider and Wolverine often busted open some huge combos on me despite the lowest difficulty. If you're losing to them, simply interrupt the battle with a second controller if you have one or try again.
The end boss is a bit tricky, unless you've got a solid trick to it. This is where picking a good assist comes in to play, I personally preferred Psylocke, Juggernaut or Colossus for this fight. If you have a specific character you like, here's a site that provides all of the codes in order to choose your assist before the screen pops up in the character select menu. http://www.gamefaqs.com/xbox360/676131-marvel-vs-capcom-orig...
Simply hold back in order to guard his attacks, all the while pressing Triangle and Circle when the attacks end. If you're lucky, your assist will punish the boss. With higher damage settings you should easily be able to take off half of the bosses health with the proper assists.
For the second portion of this fight, you'll really want to make sure you aim for the head. Personally, I swap out as soon as the fight begins in order to have Megaman ready from the start. Once the boss goes off screen to attack you with a charging attack, simply use his QCF + his two punches in order to give a beam to the face that should take off some monster damage. This works with War Machine as well, however it doesn't take up quite as much of the screen. Play defensively for the most part and jump over the green energy balls, or you'll find yourself stunned more often than not.
With a few tries, this should be as good as yours! Happy hunting.