Ahh, revenge spiders resistant to physical attacks...I gave him a wide selection of party weapons (ranging from Flail of Ages to Mordenkainen's Sword to the Silver Sword) and had him variously run away, cast breach and fight, or try to kill the spiders with physical attacks while waiting for Achryssa's spells to wind down. I found that Achryssa teleported him to her, or the spiders attacked, enough to spoil most spellcasting.
I'll try the summoned monster thing--I've had bad luck with summoned monsters in general, so I don't think to try that often. If I can get it down to just Achryssa vs Solaufein, it should be much more possible.
Thanks!