Increasing Dragon Killer damage so that successfully hitting with all three takes off 75% Vishap's hp instead of the 60% it does now? Good Adjustment in my opinion.
Uhh, right now hitting with all three only takes off 20-21% of Vishap's health; I have no clue where you pulled 60% from.


Consider this also: how many failed Steps have you seen where the group pulls off all mechanics, but Vishap still had less than 15% HP left?
Not very many. Usually when the party fails it is a colossal failure, with adds swarming and overwhelming anyone who gets close around the last third of the fight, meaning Vishap is usually left with over half his HP still (sometimes a lot more than that).