It's highly recommended you do this after you have the BFA achievement in New Game Plus.
Some key tips to help get to level 100 easily:
-New game plus can be played on any difficulty. Thus you can easily complete if you already managed to get the "BFA" achievement. You should already be around level 60 at this point.
-Before you start your first NG+, I'd recommend you have Balanced Fortune enhancement fully upgraded and attached to your whip (or which ever weapon you use most). This increases the lurcher drop by 50%. You should have Riders mercy already by saving all the humans. This increases lurcher drops by another 10%. Also make sure you complete the crucible in your previous run and get the armor that provides yet another 10% lurcher drop. (Don't sweat it if you didn't get it). This can be upgraded in NG+ to provide a larger lurcher reward. To upgrade you need to defeat the enemies with red eyes in new game plus to get a special ore you use at the Makers Forge. Don't go out of your to upgrade this beyond level +3 this run as you will get a better armor shortly.
-Each playthrough gives you 15 luminous visages. These are free level ups and are best to save for the later levels, as each level up requires more and more lurchers otherwise.
-I highly recommend beating your first run of NG+ quickly, only going out of your way to get the 15 luminous visages. DO NOT SHATTER ANY OF YOUR LURCHER CRYSTALS DURING THIS RUN. NG+2 gives you the Armageddon armor that increases the amount of lurchers you receive while shattering crystals. Also red eyed enemies appear more often and give you better ores to further upgrade your armor. Focus on upgrading this armor this run. If you saved up enough lurcher crystals I wouldn't worry about getting the armor beyond +3. When you feel you accumulated enough lurcher crystals to get somewhere in the ballpark of level 85, go crazy and shatter them all. Your 15 luminous visages from the last run will get you over the top. If you are short of level 85, again don't worry as this NG+ will grant you another 15 luminous visages. Focus on collecting the easier ones until you get to level 100.
-Finally to get the achievement to actually pop, I had to feed some lurchers to Vulgrim.
Some additional tips to note if you are still just short:
-In NG+ you will find "Wisdom shards." These boost the lurcher drop rate by another 100%-200% for a limited time.
-Certain unique drops are now replaced by "Forsaken" Lurcher crystals. These give a large amount of lurchers. Each NG+ completion grants you more lurchers from these crystals.