I ALWAYS encounter it every time I'm first going to Tiamat's room. Every single time. >_>

Anyway, the first time I beat it was in the 30s or so, after I had gone through the Chaos Shrine in the past. I got lucky and it didn't nucleify me too often, though my Knight did die, my other party members survived. I used a Knight, Red Wizard, White Wizard, and Master.