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[Absurdity]

In the beginning you can (and should) also throw in smaller CDs like arms length or Reprisal together with your bigger CDs.

I would not use arm's length and reprisal together with your 30% cd. Since arm's length slows the mobs attack speed the only thing you get out of it is less mitigation from your big CD.

Pop arm's length about 3-4 seconds before your 30% runs out so by the time you have no mitigation up the enemies are slowed and then combine it with reprisal as "intermission" mitigation before popping something like rampart.

I should also add that you want to be careful with a WHM as your healer. If they actually know what they're doing then they'll start spamming holy for a while as soon as you stop pulling more mobs. This means that the enemies will be stunned for about 7 seconds and using your defensives during that means you're potentially wasting half of your 30% CDs duration since nothing is attacking you.

generally yes u want to spam aoes until there are 2-3 mobs, at this point depending on your healer's mana just start to pull the next set of packs.

For warrior and gunbreaker it is a dps increase to use your aoe combo on 2+ mobs.

Keen Edge -> Brutal Shell -> Solid Barrel = 3 GCDs for 900 potency total, Demon Slice -> Demon Slaughter = 2 GCDs for 400 potency per enemy hit. So let's assume you do 6 GCDs with atleast 2 enemies. Your single target combo would deal 1800 potency in damage and your aoe would deal 1200 potency per enemy in the same amount of time for a total of 2400 potency in dps. It becomes a bit more complicated if we factor in cartridge spenders but generally you want to prioritise your Gnashing Fang combo for 1-3 mobs, at 4+ mobs Fated Circle will deal more dps.

I'm not too sure about paladin or dark knight but even there you can safely assume that at 3 or more enemies your aoe will deal more damage in total. This obviously changes at lower levels when you only have access to 1 aoe ability with relatively low potency.