I never thought I'd say this about the Ark, but I feel like it was almost TOO easy. My Warrior struggled with it, the Rogue had a bit of an easier time, but then the Mage pretty much breezed through it. For reference, all three character builds were at level 24 before entering the Ark.
My Mage had Summoner maxed out, with Iron Golem and Sparkling summonables available. I also had fireball maxed, though I never used it in the Ark, and Lightning Bolt maxed, and I maybe used that three or four times, since it seemed to take Steel Spiders down almost to destruction with one hit. But mostly it was just "Summon Iron Golem for Steel Spiders, summon Sparkling for Steel Wizards and Battle Droids. Then just keep hitting them with Staff of Storms. (I didn't have Arcanist until after I reset my skill points later, so Staff of Storms seems best; no mana cost, but has stun and decent dps.) Since I already had been through the Ark twice before and also had high Int and Pers, I got the peaceful ending easily, of course. I had earlier farmed a bunch of Elixers of Galade and six Epic Omeletes in preparation for the Ark, based on how the Warrior and Rogue had fared. I used a couple omeletes and recoveries, but hardly any potions.
So I'm thinking it's a HUGE, HUGE advantage to be able to summon beasties like that, whereas the other two can't bring anyone with them and summoned beasts from scrolls tend to be weak and die quickly. I haven't done the Ark with the Cleric yet, so I dunno about that class. But the fact that you can summon a puny little Sparkling and it'll tank multiple Steel Wizards and Battle Droids while you sit back and pick them apart at your leisure almost feels like an exploit to me.
After the Ark, I reset skills to prep for the SOH, especially that Elemental Plane of Fire, which always seems to be the biggest headache. Maxed Fire Mastery and decent fire equipment, had something like 160 fire resistance before using any potions. Used a potion of speed to get through the fire plane, bypassed all the demons and elementals, and just got the heck out of there. I'll probably go back and take them on when I feel like it. But again, when it came to the first level of the sewer, with the Devourers and all that, since I this time not only had summoned creatures but also Hirge, plus I had stun immunity from the Amulet of Dark Whispers and Stability from boots, etc, I hardly ever got hit. My Iron Golem would sometimes get stunned, but that was about it.
So I don't know; maybe I waited until my characters were of such a high level that I had overkilled it. But I think Mage was super-easy compared to the rest of the characters I've used.