Unfortunately the cost of an Earth Staff is simply too great for it to offset the gains you receive. Obviously if you want the Mage Epic for fashion or fun that trumps everything. This is a game after all. But you won't suddenly be able to kill significantly harder mobs.
Allishia gave me some DPS data on the Epic pet (post nerf). It does 80-90 DPS in terms of melee damage + procs, and the damage shield is doing around 30 DPS. So when the epic pet is tanking you're getting around 120 DPS, not including nukes.
Those are pretty good numbers, a Rogue with Epic and a second good weapon is doing around 133 DPS with backstabs on Sebilis mobs. The main problem Mages have is no CC of any kind. You may be able to do some harder mobs if you have bags of root nets and give your epic pet a swarmcaller, but you are going to be sinking a lot of time and plat into keeping those items on your character, ready for the next target.
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