Water Aegir (Aegir)

Most definately useful for the branding effect all around. Great for bosses with continuous damage and decent to high base damages.

It might just be me, but I think this is an underrated Barb King.  He has the great branding effect like his brothers, the same passive boosts on his 3rd skill, and the same continuous damage on his first.  The problem is people try to build him like his brothers for hight damage, and thats not going to work as well for him.  He is a fantastic support mon.  Self heals mean he doesn't need to be the target of healing, so others can get it and stay alive longer.  Branding boosts everybodies attack and stacks with def break.  Continuous damage drains hp quickly.  And his 3rd skill steals buffs.  Pair him with Belladon and every dungeon boss will take max damage from every hit, never keep any of their buffs, and you can survive for quite a long time.  A perfect mon to add to your dragons and giants b10 teams.  Build swift/focus, spd/hp/hp, with speed and acc subs to max out his support capibilities. 

The problem, I think, is that if you build him as a support monster, he doesn't really bring utility other than the brand and stealing a couple buffs from one enemy. His brothers offer so much more. Mimirr has 100% atb reduction and stun on single target. Hraesvelg speed boosts and atk buffs with an aoe hit. Hrungnir can go HAM on the enemies. Surtr I would say is on par with Aegir because his s3 is a nuke that does +50% dmg when the enemy is below 50% health. At that point, the enemy would most likely already be dead without the added damage, making it unnecessary. Aegir's s3 I would say is a little more useful than Surtr's, but it is still very situational, and there are better options for damage and utility.

It is true that there are better options out there than Aegir, I'm just trying to offer the best build I know for him to get the most out of him.  I personally only see him as a useful dungeon monster or as a solo farmer when given good runes.  I like to use him as a support mon for my b10 teams because thats the only role I see him filling well and the only role I need filled at the moment.  In pvp he can be used, but I agree that he is not as good as his brothers there.  I just believe that he greatly outshines them in pve if you are looking to boost your giants and dragons b10 teams.

For Giants, Hraesvelg is your best bet from the Barbaric King family. His s3 AoE is great for clearing speed on trash waves, and his s3 also gives the speed boost and atk buff to your team to help everyone clear even faster and give more turns in general, which makes it safer. As for Dragons, he might be alright for stripping the immunity, but in a standard team, someone should already have bella and megan for strip. In that situation, I think he'd be better on a bruiser build with 55% accuracy, so he hopefully procs the two continuous dmg from s1 more often. I'm not saying he is unusable. I just think it's easier to use other people instead of forcing him to work for a specific situation.

Who knows, maybe this is just me trying to justify the fact that I made him my first six star.  This is just the role I use him in and it works out very well for me.

I am a brand new player.  He is my first 4* and 6*  him asap.  With the little bit of research he is viable for GB10 and DB10.  Best begining build is Swift/Energy SPD%/HP%/HP%.  Why? Becuase his dmg comes from dots/brand and he needs to be alive to do that.  Speed makes it so he has more turns hence more dot dmg and reapplies brands. 
Side note:  When you post be informative.  Don't just say that there are better mobs out there.  No one has all the mobs to choose from ASAP.  You get what you get and anyone that is looking up info just wants to know two thing... is this mob viable, if so how do I build them....  Peace out SW playas