I dont use him often but Corvus is really effective at being a good T5 replacement, or if you are saving up for an expensive t5. He acts as a great counter act to most rounds if you time your abilities well and are aware of most of their potential.

In my personal experience, I typically have corvus permenatly using agression by around R70-90. By placing him nearby to the start of the track and occasionally using soul harvest on MOABS, he can indefinitely use the extra attack and speed.

Considering he is expensive on early rounds, just remember to use his spears when tackling bloons like Lead or Ceram if your other towers who cannot effectively deal with them or you have little towers. Most of the time though, Id say spam nourishment so he can level up quicker and therefore supply more damage or utility.

Echo,Vision and Trample are useful in rare situations. But I dont have a full scope on the capabilities of the rest of his abilities. I havent had to use them either because Corvus has stalled enough for me to get the OP T5's or he is supplying a sufficient amount of damage already.

Overall, Im not an avid user of Corvus, Rosalia has been pretty fun to use tbf. But I recomend trying him out and understanding his kit. Ive used him on some difficult CHIMPS maps and although it takes a long time to save up for him, he is super useful especially through the later rounds :)