How about Assassinate: Kill a creature whose current hitpoints are less than their full hitpoints and gain +1/+1.
So if the opposing creatures has been damaged?
Yep. So it has synergy with virus, retrovirus, plague, aflatoxin, bloodsucker, liquid shadow, eagle's eye, shockwave, fire lance, ice lance, siphon life, fire storm, pandemonium, thunderstorm and unstable gas (maybe I missed a few).