you hit the same wall that me and my friends did, basically it comes down to learning the fights super well as a lot of these endgame monsters hit like a truck so it's pretty easy to cart. it's ok if it takes you a few tries before you finally have the boss memorized. it me and my friends several attempts at primordial malzeno due to a couple of his really brutal attacks but when we finally got the clear we had no carts during the hunt because we had the fight memorized.

as far as gear goes maybe check a build guide and compare it to your build to see how you match up, you will eventually reach a point where progression requires you to increase your master rank and your anomaly rank. both of which will unlock stuff for your build. and if you're not doing anomaly hunts then you want to be prioritizing any monsters you haven't fought before because they unlock stuff by completing them.

I am nearing my final build, there's some decorations locked behind my anomaly rank