I found an Ekhidna Decoction to be a real help on my sword-based heavy-armour version of Geralt (with heavy use of Quen casting).
Coupled with some enthusiastic running around to dodge various special attacks, of course. I should perhaps add that dodging doesn't have to be reactive. The enemies aren't smart enough to predict movement (as far as I can tell) so if you just keep moving when you're unable to damage the boss there's a good chance any (ranged) attacks will miss you.