I love the idea of sending in multiple teams to attack Galactus at the same time. You would need to give Galactus a way to prevent winfinite combos, such as force ending your turn if he takes X damage or if perhaps whenever you you use an ability. Otherwise the 1,000,000 health Galactus that you are intended to die to would actually end up taking insane damage from exploding heads.
Attacking different “parts” of Galactus would be how I would handle it. Make six teams to send against him at the same time. When one team destroys the part or is wiped, you jump to the next battle.
Galactus’ Head, Body, Left Arm, Right Arm, Left Leg, and Right Leg would be the different battles, each with a unique ability set. Total damage dealt in the six battles would be the points achieved. Once a part is destroyed, you send in one fewer team the next time you face him. Once you have destroyed all six parts, you get Galactus’ full attention and go into the final stage where you can really do serious damage.
In the final stage, Galactus needs 30 AP to do the full party wipe. He does this through matching tiles, a Hulkbuster type move that converts health into AP, and through a Hood-like AP theft of the team’s cosmic energy. Instead of making this a 3 round battle, Galactus’ final form will take serious time and you are not expected to be able to win… just do a ton of damage in the 15-20 turns you get before he wipes your party. You know that you aren’t expected to win because he has 1,000,000 health in this fight.
The special thing about this fight is that when one character is downed before Galactus uses his 30 AP move, another character is subbed in from a three member reserve that you pick before the battle starts.
Galactus (Final)
1,000,000 Max Health.
Galactus’ tile match damage is 550 + (30 * Round#) in all colors, but only 1 in team-up tiles. His strongest color is considered Blue. Galactus makes matches like a normal enemy.
Omnipotence (30 
AP)
Galactus crushes the party, ending the battle. (Passive) Galactus is immune to stun and absorbs 1 AP of every color each turn.
Galactus’ Will (passive)
Whenever Galactus receives damage greater than 1,300, he ends your turn. He then neutralizes up to 5 random enemy tiles, if he does so, he also consumes 2,000 + (200 * round#) health from himself and all enemies to gain 2 
AP for each special tile so consumed.
Defensive Action (passive)
Consumes 4 
AP. At the start of the turn, randomly place a strength 550 + (30 * Round#) red attack tile and yellow protect tile on any non-friendly tile and the enemy team gains a strength 550 + (30 * Round#) strike tile.
He can’t be stunned, if you hit him hard enough he ends your turn (to prevent winfinite and other match five combos where the crit hits big), and if you have special tiles on the board, he consumes them to gain AP (making it tough to use Psylocke to steal his largest AP pool or using Star-Lord to hold on for a couple more turns). You can still use Hood and OBW to slow down his AP production (Loki is a huge risk because he creates special tiles), but that needs to be combined with solid damage. When Galactus consumes your health, he isn’t doing direct damage, so protect tiles aren’t going to save you.
You can steal his two special tiles, but they will probably just end up feeding him 
AP when the attack tile goes off. Better to just bring a bit of board shake to handle the special tiles… and make sure that you don’t hit him when you’ve got too many special tiles on the board, or this fight will be over soon.
The fight isn’t fair and you are expected to lose. However, you aren’t going to lose on turn 3 and you can feel really good about hitting him as hard as you can for as long as you can. As far as I can tell, the fight doesn’t have any major exploits that will allow someone to actually do 1,000,000 damage, but good teams would be able to feel good about wailing on Galactus for awhile.
At the end of the battle, your heroes are healed to full… let’s say by one of the other heroes who weren’t in the fight.