OK, I accidentally found out how to get your thrall to help kill the Abyssal Remnant.
I give all my base defense thralls a stack of javelins just in case any raiders choose to stand on top of something to avoid getting killed. I took one of these thralls to the abyssal remnant who was bugging out due to server issues but the thrall dutifylly went into the acid, weilded the javelin and took half the abyssal remnants health throwing 6 javelins before the boss woke up and let me hit it.
There you go, give your thrall a couple a stacks of javelins and it will kill the abyssal remnant.
Have fun, and feast on that delicious abyssal flesh.