Best use for saga gem ?

DO NOT go for prowler
R3 unduped and damage is HORRIBLE let's start there
His need to be able to heavy the opponent at the start of the fight or heavy punish their special attack IMMEDIATELY after they throw it sounds practical on paper but is actually very impractical and inconsistent. Especially when you have BG metas that give them power such as this one or when the opponent has limber. The first hit of his heavy attack also has TERRIBLE range. He has a very tight rotation that can go very wrong very fast especially if you don't manage to get up that incinerate at the start of the fight.

His damage unduped is also not good, the videos you see online are of him 6* sig 200.

If you take up your prowler "UNDUPED" you will not be satisfied.
I'm just here to tell you what no one else will

dr2ws

As much as I want to say Spot, he needs the dupe, so in my opinion go for Knull.