what kind of XP grind in particular?
solo leveling it's still probably doing Legendary Quests and rerolling them to be Terminals or the handful of Story quests that are easy to do. alternate green/red terms based on your level range, how easy the boss is, and how strong you are.
group leveling might put the new ICP or midtown madness combo'd with rerolled Legendary Quests in them on top, but I haven't tried it.
grinding XP for Omega points is best done on the last night of the Omega event (along with popping all your boosts) and farming either Cosmic Taskmaster or some other easy boss (DocOc or Juggs). some of the other events can give decent enough XP such as Cosmic Chaos event +cosmic rushing, or the Winter event they ran this year for a few weeks had pretty nice passive server bonus at around +100%.