Levelling To Rank 50I haven't found a really quick way of doing this apart from increasing the difficulty of missions to get bonus XP that way. The various campaign missions were where I did the majority of my levelling but when I completed some of them I didn't have high enough power to start the next. I found tarot missions to be the fastest way to fill these gaps.
There will be a sweet spot for this that each player will find most efficient. For me setting them to extreme difficulty and selecting minor arcana cards to add 15% xp wasn't any trouble and was quick enough for me. The minor arcana cards I used were Unending Duty (no deaths) +10% xp and Possession (chance of demons spawning on kills) +5% xp. With these settings I could speed run (objectives only, not full clear) five or six an hour.
If you intend to get the achievements for the season two title update you shouldn't have any problem hitting rank 50 at all.
WarzoneWarzone requires a minimum power rating of 1100 to enter which you will almost certainly have by the time you hit rank 50. Once you meet that requirement open the star map and travel to:
Tenebra Subsector > Lacaon System > Lacaon Maioris
Once there you will find a map with seven nodes, six of which will be locked. The other is the starting point, stage 1. I entered stage 1 and selected battle 1. It was certainly tougher than the majority of missions I had done up until that point. I was rank 60 and had a PR of ~1550 and almost died a few times so beware.
Tips and things to bear in mind for this mission:
1. I didn't find any medical drops so it seems you only have what you enter with.
2. You have to clear all enemies in each of four sections.
3. Each section has two warp gates that will continually spawn enemies, you'll want these dead as fast as possible but mind you don't get swamped. My approach was to kill as many enemies as I could and then focus the warp gates.
4. You will have limited space to retreat and funnel enemies towards you as a door closes behind you when you move to the next section.
5. Any build with some health regen and decent AOE should get you through this with some decent ranged for when you have to back off.
The achievement popped for me on completing the mission which took about 15-20 minutes.
Good luck
