I struggled with getting the abomination's explosions to give me credit for the quest for a long time, and with help from other players' comments I've narrowed down the cause:
When you detonate the abomination, he causes all nearby creatures to cast a spell on themselves that suicides them and awards credit to you. They cannot cast this spell while stunned.
I found that the most effective way to do this quest consistently is to put the abomination on passive and follow, and walk through the clouds of mobs to aggro them. Repeatedly hit Flatulate any time you or the abomination begin taking damage. Once you're happy with the number you've gathered, STOP using Flatulate and wait until no NPCs are stunned, and detonate the abomination. You should receive credit for ALL scourge that are not tapped to other players!
Also make sure that when you detonate the abomination, the resulting explosion won't throw you headlong into more enemies.
I normally kill an Abomination, loot the Chilled Guts, summon my pet abomination, and then kill and loot a second abomination so I will have Chilled Guts for a second after detonating my current one. In this manner, i've completed the quest in one slog through the cloud of enemies!
Fight smart and this buggy quest becomes a very, very easy 12 gold.