I'm assuming here you are soloing as a pure fighter.
With regards to fireshield, it's very easy to become immune to it if you have finished the Windspear Quest.
Red Dragon Scales ----> 50% FR
Dragon Helm ----> 25% FR
Some ring you get from the Transmutater at the Dungeon Entrance ----> 40% FR
Wear these 3 and Fire will cease to be a problem.
With regards to Stoneskin and other spell protections, you can either try to absorb the punishment( not recommended) or wait it out.
Wait it out as in run away somewhere and wait for the duration of the major spell protections to run out, leaving only Stoneskin. Stoneskin is not a major problem, most fighters will chop through the skins within 2 rounds, it's the Protection from Magical Weapons( PFMW) that sucks big time. Of course, you can keep a normal weapon to hack the mage if he uses PFMW.
If the mage goes invisible, you should most probably run away and wait it out since you do not have any divination spells( gasp! Divination can be useful!
).
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If not a Sorcerer, then a Paladin.
