New player (buff + necromancy)

I just started playing this game last week. I’ve been struggling a lot having to repeat and watch countless ads…. Until I found out I can buff my zombies! It feels like cheating, but since then I am flying over timelines.

Then I reached timeline 15 and I realized there’s so much more in this game that I could imagine so I joined here and maybe you guys can give some other great tips.