Thanks DR and SS -- I finally bested the Demon Firesage, solo, in Havel's Armour and using my Black Knight Sword + 4. It really did at least twice as much damage than my Lightning Claymore. I didn't think it would, I knew it had some bonus damage against demons, but never thought it would be so high.
Remember the whole BK weapon brouhaha earlier in the thread? Vindication for Damned Registrations !
The next guy's raping me, though. Is summoning Solaire also cheesing out? I thought this could affect the 'plot' somehow so I don't want to miss that...
I don't think summoning/not-summoning Solaire affects the plot as long as you talk with him at the right points in the story. This is the Centipede Demon right? I found that fight to be fairly frustrating due to the small space on which you can fight without falling into the lava. But you can get the ring without beating the boss (tail destroy drill again). Then return with the ring on to give you a bit more mobility during the fight.
ETA:
I don't think summoning is particularly cheesy (its a game after all), but attempting to solo each boss first is far more rewarding - sometimes summoning will result in you just watching a phantom beat him for you. If a boss is getting so frustrating that you aren't having fun, go ahead and summon. The NPC summons are generally a good intermediate step if you want help but also want to participate - most of them are basically just tanks who won't just beat the boss for you (Beatrice in the forest and Iron Tarkus are apparently potential exceptions to this)