if you went in with 4 players and lost, then you did it wrong. a 4 player team should have nuked him easily. Here's how you do it:
Fire - load nothing but DoT spells (those that damage over time). These spells include scald, fire elf, link, and power link (the last two heal you over time as well). Scald will hit all monsters, which will help take care of the minion. Dont blade, dont trap, just DoT spells, spammed at will, with wand hits in when they show up. You can use the sun spell damage enhancers, but you dont need to.
Other player(s) - load up for a big hit
End of story. The last time my buddy and I went to fight him, his fire did nothing but spam, I loaded up for tempest and a few rounds later they were both dead.
This belloq fight was designed for fire's DoT spells
Belloq is a cheater but he's still very easy to beat if you know what to do. The trick is to damage him a little on every single round, that way he won't cast the Ra. Have extremely low level wand blasts, 1-pip spells, or DOT (damage-over-time) spells ready and use them.
If you have a Fire, Death, or Ice school on your team, have them pack their DOT spells. Fire Elf, Scald, Fire Dragon, Volcano, Frost Touch, & Poison are the best bets to damage Belloq for 3-4 rounds at a time. Just keep casting them over and over. For Myth, have your Puppet minion ready and keep casting him - for 0 pips he'll keep hitting every round. If he gets killed, cast him again.
If you don't have one of those schools on your team, you can still do fine. Have someone go in with a deck full of 1-pip spells and Fire Elves to keep damaging Belloq on every single round.