While solving the mutagen puzzle helps by reducing the number of tentacles, I am not sure it's worth trying to destroy the mutagen tanks as a pure psi, because they have high resistances, are immune to criticals and require a lot of damage to bring down. E.g. a critical fireball from my mage would hit a tentacle for over 500 damage, but a nearby mutagen tank for only about 50.
Try thought control instead of metathermics, neural overload is the only type of damage tchort seems to have no resistance against. I suspect those tentacles are not resistant to disabling spells, because you can fear them by setting them on fire. So if you have locus of control, you could try just walking up to tchort and hit his tentacles with an area of effect enrage or fear, and then try to spam neural overload, though you might need drugs and a good psi headband to increase your damage output (but there are components for crafting a thought control headband laying around in the warehouse area). Also, you can hit tchort with implosion first; even with its mechanical resistance the damage will still make a dent in its HPs.
But I have to agree, it seems that metathermics got the short end of the stick in development, because the feats associated with it are largely useless and don't even work well with each other. E.g. thermodynamicity reduces the AP cost if you switch between fire and ice abilities, but that means you will remove any effects (fear, hypothermia) applied by previous methathermic spells. Meanwhile psychokinesis is frankly overpowered when combined with unarmed melee, while thought control gets locus of control, neural overload and bilocation.