So, assuming no other mana boosts, you need 12% to go from 10 tiles to 9 for average speed hero.
Without that 2% you need a L23 mana troop. With it, you need a L17 one.
If they eventually get costumes (as Tibertus does), that will give them a 5% boost, so then you need L5 or L1 mana troop.
Source: Optimal Mana Bonus Numbers for Every Mana Speed (Now including Ninja Heroes)