He seems harder than he is. That said, I really hate that encounter, and basically don't touch it with a 20 foot pole unless I have no choice at all.
Anyways, I'm sure you know how to spawn him- I organize groups to be durable versus AE's moreso than straight assault groups. There's not collision detection with the walls, so note that there's no benefit from people trying to duck or anything, the only difference in getting hit by AE's is range. BUT, his range is short, it's very possible to be casting while staying outside his AE's.
There's 2 different ways to handle spiders. One is have a couple smart bards train them all. I don't like that method because I don't like depending on any one person. Personally, I had a few enchanters on either door leading into his room just chain color stunning. The spiders aren't resistant really, and a few enchanters can lock them up pretty easily and make sure none enter and 5k you all, just provided it doesn't go on very long.
Manaetic behemoth -randomly- choses one AE to be his ae for the fight on a recycle, and one ae that will be PROC'D. The difference is significant. This means one AE will go off a couple times whenever he swings, and another is just gonna be on a minute cycle or so. This makes the behemoth fight VERY random, as you can get a DoT that is chain proc'd and just raw damage, or you could get -silence- chain proc'd, or even gas vent as an interrupt or something. Basically you should really buff all resists, but plan that clerics inside the room would be silenced/interrupted. For that reason, I think it's pretty moronic to charge with all your casters. AE ranges are mostly 100ish, so if you have clerics with extended range items, just stand outside range and dump supernal remedies. His melee is nothing. You can just dump remedies and easily keep up with ae's and his melee with a few clerics. On the other hand, if the clerics charge, chances are they are just all gonna die right off the bat, will be total chaos, and behemoth will run around ganking people with low health aggro. HOLD PEOPLE BACK. Keep clerics far outside AE, dumping remedies, keep wizards back, just getting targets, and melee rush and use offensive discs right off. Have a warrior that goes first use fortitude, to give you a few seconds to latch onto him, then evasive after (some procs fire on misses/da's, this is not one of them). Same with a second tank sucking up rampage. Behemoth has no health, so really as long as you control the fight, he just isn't that dangerous. If you don't control it, then his AE's get a chance to devestate you, and you'll all die. Just don't put yourself in a position where he will move around a lot, charging randomly, etc.