are there any strategies in defeating this guy? I just got him up on my Grove screen and he keeps kicking my butt

I'm not even on The Lost Grove, still training my max stat party, but I do have a couple things that may help, even a little.

Golden Eagle:
I notice that his fourth proficiency point is dexterity. I think it may benefit to make the fourth health or agility, as those are the recommended proficiency slots for light. He is also close to an ability rank up, so if you keep training him that should help a lot.

Merlin:
I would recommend making at least one of his proficiency points intelligence so he can deal some damage in battle too. The amount he heals himself is based on the damage he does, so an intelligence point will help a lot. Merlin is also close to a rank up, so keep training him and he will be more powerful too.

Mantis:
Everything looks good for him, maybe train him some more so that his abilities will rank up though. He is pretty far from that though.