Cyndr the Mountain Heart

How Am I supposed to beat this guy only with henchmen?

The tactic is to flag the henchmen to zig zag so the pyroclastic shot don't hit them (1 hit kill in area for most of them and for me), while I zig zag too while I put the keggs on his base.

That is way too difficult, how Am I supposed to do it without Pug or heroes?
I am already -60%

1 hit kill everything, it's insane and frustating

you get heroes in eye ...so use them, or at least you can use the flags for the first 3 henchmen. It takes a bit of trying, but it can be done with just heroes/henchmen..
Flag them out first the opposite direction or wait for him to come up closer to you then flag them away and drop.