Played on Core
Phase 1 is the same as in Tactis, as far as I can tell. It's Jon the mage + the wraith (still only hurt by nonmagical weapons) + the dragon with its level drain breath (or melee attacks, it died too quickly for me to tell) + the sword that is only hurt by magic damage + a beholder that can buff itself like the SCSII elder orb. Jon still has shatter magic.
Easy phase, beat it in one go. Wasn't paying that much attention to the monsters' abilities but Jon said bits in his opening speech about the orb draining con and the sword or wraith draining strength that sounded familiar from when I did this fight in tactics.
Phase 2 is comparatively hard. The game saves, everyone gets resurrected and healed, and teleported to the same spot in front of the door as in phase 1 (and presumably tactics). Jon appears, give his vanilla speech ('can't even prevent your companions from helping you' etc) and summons the 4 demons he summons in vanilla. However, he also turns into the Slayer (keeping his spellcasting ability).
He autobuffs with the same buffs any SCSII high level mage buffs himself with, most notably SI: Abjuration and Spell Trap. He has an imp invis/Fire Shield/Fire Shield spell trigger as well.
As soon as the fight starts, he begins casting Time Stop. Once it goes off he starts meleeing the nearest party members, and he beat my 220 HP antipaladin to death through Hardiness, so he's a melee machine. I eventually beat him after quite a few tries by having my mage cast Time Stop asap. Then when it ran out Jon refused to cast his Time Stop for some reason and I kited him while I dealt with the demons. After that it was just a simple matter of debuffing him and beating him to death. He doesn't rebuff as far as I can tell, though he did have 2x Melf's Acid Arrow in a sequencer
Once he's dead the game proceeds to the epilogue as normal.
Overall easier than I remember the Tactics fight being, but I haven't played that in years.