Sea Serpent you can get on the boat solo.

You just have to stand on the end bit and shoot the last shot into the serpent as it is about to bite the boat. The trophy pops up and you can catch it before it sinks. Takes a little practice but I can get them all the time now.

I think the only trophy I never got was the stone golem one. Normally just drop them in holes but now that the crystals make walls I've started killing them. Probably get one soon.