Best of luck! For one of the best takes I've seen on the Region G 'climactic battle,' you can check out the longest-running LJ PbP WLD game:
http://spacecrime_wlg.livejournal.com/
The Region G stuff specifically starts at:
http://community.livejournal.com/spacecrime_wlg/59872.html
The BIG FINAL BATTLE HOO HAH really needs you to read the whole section on Region G to 'get it,' as it's a conglomeration of a lot of stuff. Teaser: The party ends up having to take on Glebshayy and Gwarny'z at the same time, while the ghost PC rushes the Flame of Aranas back into the celestials' area as fast as he can, hoping to bring the aid of Cyrlebrai.
Because, quite frankly, at full power, sure, Cyrlebrai might be able to take on all the demons at once. Maybe. Very doubtful - don't forget the mariliths and more. But remember, he's been powering the whole darn region for quite some time. He's not going to recover instantly - even when he's given the Flame, he's going to be quite drained, only having access to a very few of his powers. Think of it as DM fiat - he has what he needs to save the party's ass if they need it, but not much more than that. Read the spacecrime_wlg area for a GREAT usage of Cyrlebrai in the final battle.
Man, I can't wait till my group hits Region G.
Good luck!
P.S. Another reason to read the Region G bit - you get to see their encounter with the blackguard, where the lil' kobold flies up towards a spark high up in the air, thinking it's a spying quasit - and ends up running for his miserable life from a diving nightmare! Classic D&D, I tell you. Classic.
Oh, and as for your plan to run 5 fights, Heroes of Battle style, make sure they have a little aid between fights - a hound archon cleric throws a Mass Cure Moderate Wounds or something their way, perhaps. That punishing of a run of battles, no matter what the ECLs, will drain the hell out of them.
An interesting side note. In my campaign (the currently continuing WLD campaign - the other one is leaving WLD for now, as the players are sick and tired of it) the party joined the garrison, and saw the history of The Prison - that's honestly what it is, and the celestials saw no need to make up a fancy name for it. In my case, Cyrlebrai is the
oldest celestial in there, as he's been in there since the beginning - he was the one who called for it to be built in the first place when the gods would not intervene. The party also 'saw' the capture of Lord Kraveshk, and bits and pieces of the other celestial garrisons and bits of history. It's all leading in together - it should be quite enjoyable. An epic fight, for sure.
And, honestly, I'm going to have lots of FUN with Region C, thanks to Hussar's hard work on the region and adapting it for OpenRPG. Region C is just fun overall, with all the different things in it. I'm looking forward to deciding what role Arum plays - I'd love for him to be a recurring character that turns up all over the place, or at least leaves a trail behind. After all, he left Region C, the way I see it. He certainly didn't leave the dungeon, though. So, whereever the party goes - Arum's been there first. Perhaps he's still around! Who says that Invistis is the only lich in the dungeon? Not all liches have to be evil. After all, there's definite potential for a mad lich who lives on, racked with guilt over his part in the destruction of the way of Merunda. Personally, I just want a chance to run that darned random encounter from N where the lich comes up asking for diamond dust. Really.
So, yeah. Going to run C very very soon. I just need to play with Open some more, and decide what sort of spin I'm going to put on the region. I may just have to steal Hussar's incredible idea of Nardarik thinking he's Arum. Or not. We'll see! So many possibilities...