He was battling and bantering merrily, telling Soon that he couldn't be destroyed, and then Soon says, "Yeah, then I just get someone to smash the holy symbol." Xykon pauses for exactly one surprised beat panel, then immediately flees.
EXACTLY. Rather than running to regroup and attack on his own terms at the first sign of danger, he sticks it out and fights without really knowing what's going on. He doesn't even know what his opponents are, other than the fact that they're hard to kill and that they hurt when the hit. He still sticks it out.

Xykon only became cautious when someone leveled the worst threat they could. That's not very cautious.