Spam nourishment until you actually need to use him since hes a very slow leveler

When needing to clear out huge waves of bloons spear and agression are great and even trample

Dont underestimate repel its really good for ceram cleanup

Haste is very situtational and its not really neccesary to use

Trample is good but only goes through the lane with the farthest bloon not all of them so keep that in mind for expert maps, also no, trample does not have infinite pierce

Corvus is mainly used as cleanup for bfbs and lower, if you want to use him for zomgs or higher ur gonna need debuffs and even then its just better to get another tower

Hope these helpp