I find life exchange very useful on my Nether Fairie Dragon. Pairing that with Slicing Wind works wonders on aquatics.
I don't think that sunlight benefits life exchange, since it "equalizes" the health between you and the opponent. What sunlight DOES benefit though is the % of health moves. For example:Darkmoon Hatchling
has both Predatory Strike
. If you start with Sunlight
and switch in the Hatchling, you can do 600 minimum damage with trample, a bit more against the critter legendaries. The magic number for Predatory Strike becomes 1500+.
The biggest downside for this l'il guy is that he IS a critter, and thus weak against legendary beast types. Also, he probably won't last enough turns to use both moves effectively. It would take about 5-6 turns for the hatchling to kill the opponent, assuming the first pet only uses sunlight, and no other moves.