Ben 10 Omniverse

Four Arms is a character you have unlocked from the start of the game. All you need to do is defeat 50 enemies with any attack you choose and the achievement will unlock on the 50th defeated enemy. ( His first Y attack helps finish them off easily as he will hit them airborne stopping them from attacking.

Before you fighting Malware by first time, there is a puzzle to be solved with Feedback, if you not solve the puzzle, enemies spawn infinitely, are really easy to kill them. It's possible to upgrade all the characters in that way too.