Active skill is a skill with a letter "A" to the right of it in a monster skill tree. For this achievement you need to max 5 of them.
I highly doubt that anyone have a build with 5 maxed active skills as there are only 3 slots for active skills to use.
You also need a high (40+) level monster, that probably will not be used anymore (without another reset of course) =)
I used my lvl 50 Gunslinger and maxed these skills:
- Charge Attack 10
- Combo Frenzy 10 + Combo Mastery 5 to unlock
- Slam 1 + Double Jump 1 to unlock
- Slow Motion 10
- Bomb 10 + Combo Extension 5 to unlock
Total: 52 points (out of 58 max with a 50 lvl monster)
52 - for Berserker
49 - for Adventurer
50 - for Magician