No matter how you build your pet there will come a point where it will die from a combination of poor AI and overwhelming enemy dps. In most missions you can just revive it, but in arbys it dies for good. For very long arby missions you can consider Djinn with Reawaken, it's squishy but you have an unlimited number of revives for it with a 90 second wait. And build your frame for the inevitable death of your companion, such as putting on Enemy sense or Enemy radar so you won't be blinded when you lose your pet. Vacuum is annoying to lose but you can mostly compensate or ignore it... resources (most players in such long runs won't give a crap anyway), ammo pickups (you'll likely be using just melee or abilities since your guns won't be as effective) and orbs (bring pizzas or use zenurik if doing heavy casting).