Well, at first, I didn't know that we could attach spine crawlers to a unit, and I haven't seen the leviathan at all. I more of wanted to do a vine whip attack, but I couldn't really think of any projectile that looked like it.
Anyway, for now, I want to fix what I'm so close of finishing. I already have all the actors, missiles and stuff mentionned in the tutorial, I just don't know what might have gone wrong. Even trying to go compare the tutorial or even its map file and what I've done, I am unable to find the error.