Furnace Golem (like any tall/ giant enemy) can be cheesed with the Pest Threads incantation. The miracle can be purchased by Sage Gowry for 7500 runes (process the Millicent questline till he offers it).
Doing it while on torrent makes it take less time because you can cast wayy quicker, check out the video on my page I do it on horseback and it’s super fast
I found it easier to just keep using pest threads. With pest-thread spears (new version of it in the DLC) I could kill the golems before they got up again.
