Indeed it's hard to answer this question with some proofs or sources, since all we can find about it are quite poor statistics made by players. But from my own experiece of killing different raids or bosses: there is much bigger chance to drop Silver Raid from a boss, than from a regular creature during raid.
I have heard a lot of rumors, that it its easier to loot token from a stronger monster, than, for example, from a rat during invasion on Thais, and indeed it seems to be hard during this particular raid. BUT! During killing other anounced raids with a regular creatures I didn't noticed visible difference in probability of looting Silver Raid Token .
Anyway, as long as Cipsoft won't publish any official information about it we can not be sure.