These days if you want to get apexis crystals you should run the Tanaan Jungle daily quests. Those, combined with the crystals mobs drop, will give you plenty.
However, Assault on Skettis still has a use – the Time to Strike weekend event
Previously I’ve done this quest by doing Tanaan dailies; at the time I was going to do them anyway for the reputation after all. But now? These days I don’t go near Tanaan Jungle at all – I’ve grinded the reps and its only hanging around for Doomroller and friends that I’ll go there!
So I returned to Assault on Skettis as with flying you can do all five of the apexis areas in ~25 minutes or so.
After purchasing the quest from the Sargent, fly to the northen part of Spires of Arak, roughly where the yellow dot on the map below is:
If you’ve got Draenor flying then you can fly pretty much direct to the area:
This is the area you’re looking for:
You can land in the middle of the Arakkoa prisoners; you’ll probably need to kill the pair of guards who circle them.
Once there, follow the instructions on Accumulating Apexis – free four outcasts and then kill mobs. Fortunately there are lots of mobs nearby to kill!
Hand the Outcasts over to the Outcast Darkscryer & fly back to your garrison to hand in.
Then repeat another four times!
In the time it takes you to go back to your garrison, buy the quest item and then fly back the mobs will likely all have respawned again.