I really need help, I've tried a few times and I cannot seem to beat the level. The farthest I've gone was having killed the 2 weak AI's, but the third, most powerful AI, overpowered me to due to his tons of armies in territories, like the AI had a stack of 230 in one of his territories...

Any tips to beating this level? I gave up and beat the level after that, but idk what else to do.

I've not tried that level yet, since SP bores me, but I've done some googling and found a few things you could look at that will probalby help you out:

Video of a 21 turn game (gold star) with tips and tricks on how to beat the levels:
https://www.youtube.com/watch?v=oRTqwuORc28
Key points that the video points out in its title screen:
1.   Dont allow your 2 main frontline enemies to get stopped at any turn, otherwise you will not reach AI-4 in time to stop them from diplo-ing you.
2.   You dont need the diplo cards, but it might help you save some armies you need to attack other AI's. Make sure you dont violate point 1.

A forum suggesting you dont use the diplo card at all:
https://www.warzone.com/Forum/164336-single-player-serfs-up

Video of a 45 turn game (it exists, but this is probably not really good, since it should only take ~25 turns):
https://www.youtube.com/watch?v=RKYq13yJJmk

For what its worth, this is the level in question (for anyone looking at this later):
https://www.warzone.com/singleplayer/level?ID=81

watched the video, got the inspiration, used my own strategy after turn 8, and won in 31 turns. that video helped.