I figured out the key to beating this level with a gold star. The yeti will always attack an AI opponent if there is one neighboring it. I don’t know of any other map where the AI general has this behavior. This behavior only applies to the yeti, not to the armies of the yeti. Leave the non-yeti AI opponents alive such that the yeti follows a path to a choke point where there are only two possible moves for it, which will likely be the south island, unless you can trap it a location where there is only one possible move. I took the east, then the north, then the west coast, such that it followed along the southern coast. As it progresses towards the south island, conquer all the Yeti-owned areas behind it, such that when it arrives on the south island, you own every territory other than the south island.

Move or airlift all your armies to the two sides so that it cannot escape.

Also helpful is knowing the “Reinforcement Card Schedule” in the post by Dogman https://war.app/Forum/433174-yeti-strategy-gold, so that attacks on yeti’s territories can be delayed on turns when its reinforcements show up. The yeti’s armies always seem to attack, even against larger armies, so it is better to wear down those bonus armies by letting it attack first when possible.

In my case, the Yeti arrived on the island on turn 23, which I had left as the last remaining non-yeti AI holding. The Yeti had a strength of 203. At the start of round 24 I had 149 armies on one side at 159 on the other, so it could not escape. The battle was won by the end of round 25.