1. The Warlock doesn't really have anything solid that deals with incorporeal stuff
2. Yet another reason IMO that Lightning > Fire.
1. Demonhide Armor is a safe bet. Every mage needs armor and one that deals ethereal critical damage pretty easily solves most incorporeal problems. Against the Spirit, though, just Enfeeble it. You effectively solve a 12 mana (plus upkeep) creature for 6 mana, since the only real reason to use the Spirit is for its quick action melee attack anyway.
Ah yes, Demonhide Armor, I had forgotten that it did Ethereal damage. That's one option, although I'm not a fan of the card personally.
I don't get the Enfeeble comment. The Enfeeble curse makes a creature Slow, but that doesn't necessarily prevent it from being a pain to deal with. One of the more common uses of Whirling Spirit is to place it in the same zone as its controlling mage so that it can act as a Guard or just attack normally and knock melee mages out of the zone to keep them back. In that scenario the Whirling Spirit does not need to move, so being slow would be irrelevant.
2. Air spells aren't as efficient for dealing damage and all of them are avoidable, unlike most of the Fire spells. I value the unavoidable trait more highly than the ethereal trait, since I come across defenses more than incorporeal things.
No, I know that Fire is better at just doing pure damage, but I feel Lightning is better mostly due to the fact that Daze/Stun is often much more useful than Burn. As far as your comment about Unavoidable goes, fire only has one more Unavoidable attack than Lightning does now that Arc Lightning has been released in the expansion so unless you are going Firestorm heavy, you aren't really getting many more Unavoidable attacks than if you used Lightning.
Guarding and its attack are both quick actions, so the instant the Wizard needs to move (or more likely, is pushed) the Spirit becomes pretty lackluster. Even still, as a Warlock, I'd be pretty content sitting a zone away and firing off Fireblasts and Fireballs. The object isn't to kill the Spirit, it's to stop it from guarding and quick attacking, which Enfeeble does pretty effectively.
I have the base set, and from what I can tell, the only avoidable fire spell is Fireball, which is still more efficient and has a better chance to afflict a status than the comparable Lightning Bolt. The three other fire attack spells (fireblast, firestorm, ring of fire) are all unavoidable, so assuming there's no unavoidable fire spells in the expansion, that's still two more.
There are also ways to get around daze/stun with attack-like incantations such as Explode and Drain Life. You can also burn conjurations, which I think is a pretty big deal.
