I killed it with my alpha desert titan, believe it or not.

I killed and tamed the desert titan first, and moved onto the forest. I wasn’t having luck with giga taming or breeding so i just decided to use my desert titan.

The lightning attack was my initial idea, but that doesn’t work on the forest titan. But the bite does a good amount of damage. Just need to find a position where you don’t get attacked often, but can still bite it.

Definitely not ideal, but it’s doable, if you happen to be in the situation.