You can stop Oaks spirits by upgrading the towers, easy enough.... Also if Oak is using skill points for spirits and not keeping them with him then he's at a disadvantage in that he's not benefitting from them. You should be able to push him around a lot easier.
As for Crucible, it can be annoying when someone does that but it's pretty easy to defend against. Teleport back and kill him, or at least force him out of your base. And while he's not on the main field your teammate can push.
Any general can do this with Cloak of Night, though it is really fiddly as you have to get exactly the right spot.
