Here is the full list of all 39 99Vidas achievements.
Finish the game with 2-4 players in co-op, either local or online
Beat the Final Boss (Final Form)
Beat the Final Boss
Beat the Fifth Boss
Reach the Second Bonus Stage
Beat the Fourth Boss
Beat the Third Boss
Reach the First Bonus Stage
Beat the Second Boss
Beat Miss Grace
Beat the First Boss
No germs are fast enough to keep you from grabbing a bite from the floor
Find out there's no such thing as 99lives
Finish the Game without using Magic (Super)
Beat 99 of the same enemy type
Win a Match on Versus Mode (either Local or Online)
Acquire all upgrades for King
Acquire all upgrades for Izzy
Acquire all upgrades for Fries
Acquire all upgrades for Oak!
Acquire all upgrades for Hannah
Acquire all upgrades for Bebba
Acquire all upgrades for DeeDee
Acquire all upgrades for Rey
Acquire all upgrades for Mike
Acquire all upgrades for Dan
Acquire all upgrades for Sadiv66
Finish the game with King, Izzy, Fries and Oak!
Finish the game with Hannah, Bebba, DeeDee and Rey
Finish the game with Mike and Dan
Finish the game with Sadiv66
Finish the game with a couple
Finish the game with Izzy in the Izzy Difficulty Setting
Finish the game in the Normal Difficulty Setting
Finish the game in the Hard Difficulty Setting
Finish the game in the Elite Difficulty Setting
Use a total of 99 lives in game (cumulative through multiple playthroughs)
Acquire all upgrades for all characters (unlockable included)
Prove you're worthy of the 99vidas by unlocking all the Achievements