Okay, if the soldier is completely made of water, how exactly are you supposed to burn him?

Regarding mana, yes, it should be 22 so that he can cast a spell once. If he casts too many spells, it will actually be detrimental to him.

How else can you portray a large water warrior with a trident if he's going to be weak?

He's a melee unit with not-so-powerful attack and stats around level 7, with increased defense. The demonic sword is weak; even if there are 50 or 100 of them, they will only kill 2 units with 2 hits, which isn't a deadly blade.

Regarding the number of abilities, most of them are standard elemental immunities. In addition, there's just protection against Sandro's cloaks.
The griffin's leap with double attack isn't that great. Channeling only works when used by a human; the AI can't do it. The other abilities are mostly defensive, but shouldn't it be easy to deal damage to a water creature? Knight's Charge is quite justified, like a wave coming in, and the same goes for the stun ability. If you also add an animation for his movement that looks like a tsunami, it would be even better.

And level 7 makes him vulnerable to hero attacks. At level 8, it will be even worse. And the proposed stats are definitely not for level 8.