I hesitate to recommend the following, which would be the way something stuck like that would be removed.

Normally, I'd say take a cold chisel and put it on the inside and whack it with a hammer to drive it--but one slip and you'll damage your liner.

If the through-wall fitting is accessible from the outside, you may just want to lower water below it, and remove the entire thing, then remove the fixed part of the eyeball. I'm assuming you can't get slip-jaw pump pliers on it.

You may try to drive a large socket like a spark-plug wrench with hexagon points on the outside straight into it, then just unscrew it that way.

In any case, plan on the piece you are removing being a throw-away.