Time to think outside the box.
D:OS2 is open world. NOTHING is forcing you to do anything in any specific order. Have you tried delaying the fight? If you have been good about getting at getting XP, you should be level 13 or 14 for the fight. HOWEVER, at level 13, you just need a character with good persuasion to talk to the elves to get the quest to save Seheila. Now you just need a character with good persuasion to talk your way into the lumber mill with the Lone Wolves. Pop spirit vision. There are several quests you can do without getting into a fight. These should give you 30k XP which should get you to level 14 if you weren't already there. Some other nice free XP can be attained on Bloodmoon Island. Avoid demons, the Advocate and the tree in the middle. Make you way to the north-west corner of the island. Use Spirit Vision to find the Druid who will tell you about the academy on the island. Hop down into the academy, loot the place and pick up an extra teleporter pyramid and some free XP. At level 14, gear up and clean out the Lone Wolves. As long as you are careful not to proc them all at the same time, they aren't too difficult. Get Saheila back to the elves and then head out the back side of the lumber camp to the caravan. The only difficult fight in the area is the one with the creature causing all the decaying. The rest of the fights are fairly easy. Start the quest chain with Almira. This should get you up to level 15 or close enough to level 15 that either of the demon fights on Bloodmoon Island should get you the rest of the way. Now gear up to level 15 and go back to the pits and murder those puny voidwoken AND keep Gwydian alive. FYI : As an example of delaying things to a later level, my last couple play throughs I have skipped Wreckers Cave until level 14. That way I can kill the voidwoken that ambush you and split your party up. It is only an extra 15.5k XP but the challenge of it is fun and I enjoy getting away with something I shouldn't.
Other things I have found that help are to teleport magisters down to the bigger voidwoken to keep them distracted. Have some teleporter scrolls ready if the magisters try to keep getting back to the top of the oil rig. Cryogenic Stasis is a good way to prevent Gwydian from doing something stupid for a round. Feel free to cheese the encounter a bit as well. Before combat starts, buff Gwydian. Another cheese that can help is to put crates at the top of ladders so magisters can't use the ladders. It forces them to deal with the voidwoken, which is basically doing some of your job for you. In short, do your best to make it a two way fight instead of a three way fight. If both you and the magisters focus the voidwoken, they are annoying, but don't gang up on Gwydian enough to kill him.
Worst case, just chain Living on the Edge on Gwydian. It has a 5 turn cool down, so if you have 3 characters that take turns keeping it applied, Gwydian will be effectively immortal. I haven't had to resort to this, but it "theoretically" it should work.