For me, mana troop isn’t as prized much more anymore compared to the crit troops. They are valued only for the mana tile reduction. Since average heroes fire in 10 tiles, you only need mana troops to level 23 for these heroes to fire almost like a fast hero. Slow and very fast heroes benefit already from the level 23 mana troop. Fast heroes are already quick to charge their mana bar that reducing a tile is not commensurate to the time, effort and resources needed to get a mana troop to level 29 just for them to shave off a tile, unless you spend thousands of gems summoning in the troop portals just to get feeder troops (I don’t buy gems just to summon troop).
Ninja troops are the ones ideal to get to level 30 since they offer the highest attack multiplier on a consistent basis as well as bypassing the enemy defense buff and even riposte.
Crit troops are ideal for supporting the tank and other glass cannon heroes.
Just my 2 cents.