Realm of discord is quite hard due to amount of bosses and monsters that don't usually spawn too often during regular gameplay. If you want an easy leveling experience, I recommend you go through the game normally, especially if you're playing on elite. Then you may choose to play Realm of discord or maps, now that you unlocked some tiers.
Warlock is very easy to level with netherbolts. Anything else is up to you.
Regarding end game build, you should check Miothan's one proc build in warlock build forum. It's pretty much the only 100% efficient endgame build.
However, if you want even an easier leveling experience and have a chance of actually building other viable builds, you should try to use LAO drop mod. It enhances the 2handers in this game (and all items for that matter, but mostly for 2handers since they are underwhelming in synergies lategame) and since warlock's spells are mostly based on weapon damage, 2handers are the best option for most builds.
A build I wanted to try was one that uses nether spikes as primary spammable. It is quite strong, however I tried it on LAO mod and I can tell you endgame (darknessfalls) is quite hard. Another build you can try is to use first skill from fire tree (forgot its name) since it allows life stealing modifiers so you'll be able to recover extremely fast lategame if you have good damage and at least 30k hp (quite unrealistic, but darknessfalls is pretty hard on melee characters that don't have 90k absorb shield - engineer).
For the one proc build you need a lot of focus and for weapon damage builds you need strength and some dexterity, so be careful what you want to put your stats in cause you can't unlearn them.