Beating arcade mode with all characters is more of a grind than a challenge, considering the only thing that counts is whomever lands the final hit on Galactus. There is one additional ending that stumped me, however a friend brought it to light that you must lose and allow the countdown to reach zero upon reaching Galactus for a unique ending. This ending doesn't apply to the achievement however, as I still had it left as a ??? at the bottom of my roster. This ending does however apply to the gallery completion, so you may as well knock it out while going for this.
I'll list a few tips here to help you quickly breeze through your arcade runs. For starters, it's obvious that you'll want to be playing on very easy. It's not necessary but it speeds things up. Secondly, you'll want to set the timer down to 30 seconds. You may think this crazy as defeating Galactus in under 30 seconds would be insane, thankfully the fight with him stays at 99 seconds regardless. I also cranked the damage up so the fights were over before the 30 seconds in most cases, however this is totally depending on if you feel comfortable beating your opponent quickly. On Very Easy they aren't very aggressive, but it's your call. The only character heavily at risk using high damage is Phoenix, but you can use your first two characters to weaken the boss and simply have her land the final hit.
When choosing characters, I always wanted to have at least two characters I hadn't done leading off and a third in reserve that I was confident with. Wolverine is a simple character, I'd suggest him for many newcomers as a simply Quarter Circle Forward + B does wonders, especially if you tap B to add on more hits. Simply move through the rows or columns one at a time, picking the next character and then your comfort character afterwards.
Below I'll list the order that the character endings are in, so you don't have to think too hard in case you miss one. It's worth noting if you have the two DLC characters installed, you DO have to complete them for this achievement.
Ryu
Morrigan
Chris
Dante
Felicia
Chun-Li
Trish
Amaterasu
Viewtiful Joe
Tron
Wesker
Arthur
Spencer
Zero
Crimson Viper
Haggar
Akuma
Hsien-Ko
Jill
Wolverine
Hulk
Iron Man
Captain America
Deadpool
Doctor Doom
Super-Skrull
Thor
She-Hulk
Spider-man
X-23
M.O.D.O.K.
Phoenix
Magneto
Storm
Dormammu
Taskmaster
Sentinel
Shuma-Gorath
"Bad ending" Aka lose to Galactus.