Is there a trick to defeating temple guardians? In all the time since I've been on here I've fought & lost to most of them, even when I use my best stated pets. 

No real trick, just need to have pets with good genes, and GP helps too. :) The genes of guardians seem to vary too, so some may be easier to beat than others (though there's no way to tell this beforehand).

I've yet to win one. But my closest battle was fought with a GP pet. I imagine certain traits like Regeneration would be useful... Good luck!!

I used to win them fairly regularly, and honestly it's not worth the trouble. The last few times I've beaten any, I've gotten nothing but worthless schemes! I usually choose not to risk the energy loss now.

Other than that, it's basically just luck and a really good pet.

I've fought five or six, and I've beaten one. I got a nice expensive blueprint which I can't use yet but am happy not to pay for, and some silver. But, after I learned that fighting a temple guardian and losing automatically expels you from the island even if you have some energy left, I've decided not to fight them unless 1) I've already retrieved everything I can from that specific island, 2) I actually have a good chance of defeating it based on it's level, and 3) I'm either already at less than five energy, or I have more than five energy beforehand.