Rune Chain Spell not working - after selecting friendly troop

[asmutny]

maybe it takes out some damage due to the target's magic resistance?
I'm running with dragons and some snakes (for stone skin spell needed to upgrade Guardian Angel medal) and I'm using this spell on snakes, which gives me 3 runes = +164% damage (+55% rune dmg from the spell itself). so, the spell itself gives 1550 dmg and combined with runes - ca. 4700 dmg. so it's working correctly. I have 6245 build (updated to 6246, but too far into the game to restart).