Earth tank
Light healer
Air cannon

I'd be surprised if it's an issue with my team. I feel like it's got to be something I'm missing or doing wrong.

I agree your team and gear look pretty good. :)

When do you start attacking the plasmas? And are you getting buffs on your pets during the 1st-2nd times the behemoth summons the plasmas?

The behemoth occasionally hits all my pets with an aoe like tail lash but I'm finding it doesn't happen often if at all in a lot of my fights (maybe random or maybe when you do something you shouldn't I can't tell). The behemoth will hit your middle and side pets with vicious vines every other time he summons plasmas starting with the first summon when the plasmas have 3 stacks of the mitigation buff. It last for 2 turns. I'm not sure blocking helps but I block with all my pets when the plasmas get to 3 stacks and then at 2 stacks during the summons where the behemoth put vicious vines on your pets.

I've been attacking the plasmas pretty much as soon as they come up, and I try to get the buffs when they match up. But I'll try the blocking technique and see what happens. Thanks for the tips!

No problem good luck! I personally do not have good luck at all attacking the plasmas right away (I always wait until they have 1-2 stacks of mitigation before attacking). I'm not sure but when I have attacked them too soon it seems like the behemoth gains more mana and summons the next round a lot sooner which makes it really tough to do any damage to him. I could be wrong though.

Oh and a few other things I do differently than Artemesia that you could try if that method isn't working for you.

Early in the fight I don't use decimate on the behemoth, instead with my air pet I'll do attack #3 followed by #2 to save mana but still do damage (since decimate doesn't do a lot of damage when behemoth is close to full health). I also typically don't use divine balance with my light pet when behemoth reaches 100 mana (if the earth tank has behemoth taunted I won't block with all pets at 1 stack of recovering at 100 mana (still block with the earth tank though). Sometimes my earth tank doesn't make it but I prefer using an exceptional grove drought to revive the tank and let the light pet keep more mana so they can use #2 and #3 heals on the party or earth tank when needed. If I can't get the yellow buff on my air pet by the time I've defeated two rounds of plasmas a lot of the time I'll let my team be defeated to hopefully get luckier with buffs my next try. Also if my air pet has the yellow buff and the blue debuff at the same time I'll not try to get enough mana for a decimate and will just try to see if I can get enough for a #3 then #2 attack with air.

I just went into a particularly irritating round where my pets had the blue and yellow debuffs, but Behemoth kept summoning pink plasmas. Do you know of any way to deal with this, or is it best to just abort the attempt and try again until they line up better?

I'd personally abort if you get unlucky like that with the buffs/debuffs. If I don't get the yellow or at least the blue buff on my air pet by the second round of summons I usually let my team be defeated. It'd be interesting if there was a way to influence the color of the plasmas that show up but so far they seem random to me.

Wanted to post my guide here. It's not complete. I need to finish adding more information & images. But I think it'll help you. I was able to help som figure out the fight and she finally did it :D

https://www.sylestia.com/forums/?thread=83266

There are small bits of information that will help you be more successful at the fight. Hope it helps!
Also I talked to Krinadon about Mo Madness closing on the 2nd of december, and he'll be leaving it open till the 15 since its release was delayed! So we got lots of time to get more tomes and enigmatics. Hope to see you all trying it more :) I'm almost at capped sprite branches lolo ...should find a way to spend them :D

*whew* I finally beat the Behemoth! I got lucky with buffs; I got yellow and blue on my air, and blue on my earth tank. So nice! :D I was able to use thorncoat and sneering howl a couple times because of the buff, which I think really helped too.

One thing I started doing, is for the first 2-3 turns after the plasmas show up, I still attack behemoth because the plamsas have the mitigation on them. That seemed to help keep the behemoth health lower.