Implosion should become a part of the regular rotation whenever 2 or more targets are stacked together. You should cast Implosion once every 2-3 casts of Hand of Gul'dan to maximize Soul Shard efficiency. The damage that Wild Imps do when they Implode is based off of their remaining Energy on the primary target, while secondary targets always take full damage. Please notice that Implosion does slightly more damage than letting Wild Imps cast their 5 Firebolts onto a target; however, beside Explosive Potential cases, you should not use Implosion single target, unless the enemy is about to die. Use that global cooldown to cast a generator instead.