I always head for Oree first, but that's your call. If you intend to head on downwards, depending on your current wand status you might want to stash a wand of slowing and a few teleports along with a healing potion or two on GC1 in case you lose your wands and potions on the way.
Going down you'll want to have at least a few scrolls of detection with you, or alternatively very high INT with ESP. The mage room is completely capable of ruining your day regardless of your current gear, so you'll want to be able to skip it should shit hit the fan. Oree himself is piss-easy, getting there alive and clearing out the casters on the level safely is the hard part.
With enough AGI and DEX (around 30+) and sources for both slowing and haste, Petrus isn't very hard to take down if you take it slow and never let him hit you. Never take your chances head-on, and be mindful of the AoLS he's wearing as death clears status effects. Clearing out Attnam before engaging is very helpful.
Or of course, if you've yet to complete the Elpuri ending, that's also something you may wish to take. Being able to say you've won in IVAN is already worth it.