Current war defense: Poseidon-Fenrir-Telluria-Gefjon-Kageburado
Playing in a top 100 alliance, new there, in war I meet only monsters 
Fenrir is at +6, Fighter emblems are at Poseidon and some at Kingston. It breaks my heart to take emblems from Kingston.
Cobalt is almost maxed and I could take emblems from JF (who was the red in defense before Gefjon)
Other blue hero maybe suitable for defense: Ariel +18
So: should I replace Fenrir with Cobalt?
I personally consider Cobalt the strongest blue to place on defense at the moment. Yes, even more broken than Finley.
I have Fenrir on max and imho he is underwhelming not only on defense but overall. Use on defense is extra bad as you can never rely on the AI to fire him at the hero with <50% hp. And if he fires at hero above the threshold what he does is basically a tickle. Not even the self-healing can make up for that.
Cobalt I believe is unpredictable. Computer decides beforehand at what charge level he will fire. Anybody has him in defense?
At any speed he fires it is from good to devastating, he also directly counters Mitsuko and any other form of counterattack or any defense buff you want to use before he fires. I fought against him few battles and if I didn’t manage to take him before he fires at his 2nd/3rd tier it was really difficult to win.
I just got slaughtered by a Cobalt/Zim combo. 1200 damage to every hero. Made me a bit angry about the reflection nerf.
Yes, it’s random. But even the single charge is strong.
Ok so talent wise I should take the attack path?
Cobalt for sure. Hate hate HATE facing him. Fenrir is strong but he often attacks heroes with a lot of mana so it’s like hitting with a pillow. AI is not Fenrir’s friend.