He can probably push you to 25 without being booked, though it will be slower and more RNG dependent.  

IMO he's good enough in multiple areas of the game he's worth fully booking anyway.

YES

he will work ubooked but its more rng and random times on ur runs, can artak carry you to 25? yes, i run stage 25 with a 3 man team, artak with deathknight and skrank but yes my artak is booked, flawless 2 min runs and i can take 2 chickens to, is artak worth books? hell yes.

What short said.

You'll use him in Hydra, Spider/Hard Spider and likely Hard Ice Golem. Artak is just an absolute beast.

And what blessing do we run on him Davey.  😉

Inquiring minds want to know.  There are 4 stars waiting for this decision.  

loooool

If main focus is Hydra: Cruelty if you don't have another good option for AoE Cruelty at a reasonably high blessing, but Brimstone if you lack that (at all) is fine. I don't love Brimstone on full AoE champs, or don't view it as much of a necessity. 

For duos/hard dungeons I do believe you will want Soul Reap.

Opinions may differ but I don't think Brimstone is going to be super necessary for most people. 

I dont have an a0e option for cruelty other than Artak.  But, all of my dungeons and DT bosses need a lot of help.  Is it flip a coin to decide?

I think I'd be a fan of Brimstone in your case.