I'm noticing a theme in my soloing bosses. That theme is Efreet. The 90% debuff to enemy attacks keeps me alive, since I'm not using much healing.
On several bosses, what I've been able to do is load up my sideboard with shield treasure cards. This is pretty good for being able to stack a bunch of blades and traps until I'm ready to fire off my Colossal Efreet, and with a prior-cast fully-pipped Backdraft, I'm typically hitting for around 45,000 damage. I don't add traps or blades into my sideboard, because I'm going to need to reliably draw shields. I'm utilizing the Empowerment aura to generate pips fast enough, and along with Power Link for a boost to my life, I generally can dispatch his minions fast enough, hold the boss off, and finish him off with a couple hundred life to spare.
Belloq though can't be shielded against. He has the ability to cast Hydra or Chimera or Spectral Blast, which requires all the other schools to defend against, but his biggest hitters have no shields to protect me. The strategy then changes. You don't build up for one big hit (and luckily he doesn't heal). You load your deck up with as many Efreet as possible, as well as Power Links for equal damage and healing, and you trim out extra traps and blades that will delay putting the Efreet debuff on him. Backdraft? Too time consuming and expensive. Elemental blades and traps are just going to delay your Efreet or Power Link. Power Link in itself is a bit of a risk. When I faced him, I died the first time; but since I killed his minion, I was able to use the instant port back (through the Dungeon Recall button) and handle him fairly easily the second time without having to worry about the minion's extra damage.