I never played Crucible before and decided to give it a try, but i have to grind it first, to unlock Gladiator.
Now i have a BA DK build that should be able to clear waves 150-170 reliably, but i’m not sure it’s AoE is big enough to grind the first 2 difficulties without frying my brain due to boredom.
Taking this into account, what are good, more or less beginner friendly, builds to do it fast?
Anything with a ton of screen/crowd clearing AoE gets my vote. Some kind of caster or caster skills like Judgment, Canister Bomb, transmuted BWC, Mortars, Ravenous Earth etc. as examples. Even melee or ranged can work decently as well with some inherent AoE and preferably some mobility like Shadow Strike, Vire’s Might, Primal Strike, Fire Strike.
Your BA DK should be more than fine, you could even run up to 150 in one sitting on Gladiator using the tributes you earn from completing waves but not cashing out to buy 3 or 4 buffs to make it easier.
Note: you can not transfer over crucible checkpoints. There are no “waystones” as with SR. The only thing you can transfer over is the ability to start in crucible gladiator via the merit you can buy 
Even if you get to 150 gladiator on one char, your other chars must start from wave 1.
technically you can easily access to wave 150 gladiator by playing online with another players who have already unlocked wave 150 Gladiator. when they start wave 150, you’ll automatically get access to wave 150. you just need to farm points to do it yourself next time
Oh, that’s good to know!!! Thank you!
What about Word of Pain? Is it possible to make a WoP build capable of unlocking gladiator?
I’d say yes, though it’s not going to be entirely Word of Pain. You could easily pair 1 or 2 other damage skills with it, even if they’re component/item granted skills like Chain Lightning/Stormfire or Obliteration as examples. Procs will also make up a decent portion of damage as well since you’re debuffing entire crowds constantly.