Try this [1.1.2.5] [caster] Beginner's/budget acid Sentinel
perhaps change devos to include turtle and/or giants blood proc for HC.
HC is about your playing skills and knowledge of the game also - the build is not the only thing you can blaim if you die.
co-op definitely make things easier, but there are many stupid ways to RIP in HC MP (aura damage bugs etc) make sure you read up on these things before you guys start.
Another option is my Acid Aegis sentinel build for beginners (soon done with that, I had it posted here before but I am revising it now)
Another option is the my Vitality Cabalist, but go with this kinda offhand instead Leafmane Horn and change devotions to acid oriented
you can find my guides here ☄ Collection of malawiglenn's Beginner Friendly builds