i used quite simple tactic,but in later game quite pointless...cause every game seems repeating the same thing.
used the same tactic and reached 28.
try to upgrade your gold,and mana refrest rate..
I only used ninja..
in every game,summon some ninja,then upgrade the mana boost.
remember not to upgrade your mana capacity.

yes,try max it,and you can notice that your mana regenerates very fast.
however,the effects lasts for that play only.

the mana needed to upgrade the mana booster is always halve of the maximum capacity, so if you want to use the way i played,remember not to upgrade mana capacity in early stages.
btw,after gotten veteran ninja, finishing the enemies is actually quite easy.

it's just spamming units and upgrading mana booster.
basically i can finish every level within 3 mins.
but it isn't that effective after lvl 28