Simply put Strongest Combos are Passive Combos.. Intsakill or % health removal is nowhere near them. And since Old Legendaries combos most of them have 2-3 of them it's kinda wasted Combos.. No one will use insta kill as it doesn't offer anything... It can't be used in Arenas it can't be used in Events what's the point of having them..

Changes can be done Have only Instakill (Which can be changed to conditional Procs (Part HP Removal.. Permanent Attack decrease.. % HP removal) and Limit it to one per deck... but make it guaranteed Proc..

There are lot of ways to get this done.. Instakill either needs to be removed or given place in the game