AereA

Each character unlocks their last skill at Level 30, so you'll need all four characters to that point to unlock this achievement. You actually are given Level 1 of each skill as they unlock, so you don't need to "buy" anything for this achievement, and it will unlock as soon as the fourth character hits Level 30.

Use the second Lava area to level up. For some reason, the enemies here give way more experience than anywhere else in the game, despite this not being the last area of the game. The molten lava monsters are by far the best experience in the game.