Lately I've been traveling with Rex to complete his companion quest. But after the quest was over I immediately ditched Rex because I play on hardcoe and Rex keeps dying. He sees a swarm of Cazadors'? Bam, runs headlong in it and dies within a few seconds. Every time I see a group of enemies I don't have time to think how to kill the enemy but I have to focus on keeping Rex alive.

Anyone got tips on how to play with Rex? Seems nice to have a dog companion (like in FO:3) but he's just weak and stupid compared to the others

Well, it's a dog... Not exactly a deathclaw killing machine. You can't give it armour or additional fire power so there really isn't much u can do other than not playing hardcoe or using Turbo when fighting the likes of deathclaws to insure they're dead before Rex reaches them. I also only do the quest and then keep it as a pet at the 38, I even got dog bowls in there now and give him a nice fresh brahmin or bighorner steak every now and then.

Oh yeah, and use stims, lots and lots of stims when he gets stung by cazadors. I dunno but he also seems to die faster than other companions when poisened.

Do you have another companion or just rex? Heres what I recommend if you know if there is cazador's try to stay out of that area or snipe them from a long distance or kill them with a rocket launcher/grenade before rex gets over there. Also another thing I can suggest is tell rex to keep his distance. Oh also just run in their first while there focus is on you instead of rex but this is not recommended especially on hardcoe but best bet is to just try and avoid the cazador areas all together.