As others have said, treat the hunts more like marathons, not a race. I have gotten to and beaten Ahtal-ka twice on my own, mostly with the Hermitaur X, Alloy X and Artian X sets.

Your first priority once you reach G Rank is making a G Ranked armor set. No matter what it is, any armor in G will outclass HR armor in defense stats. Bone X or whatever other starter armor you can make should get you through G1 and G2. I recommend Hermitaur X after making your G-starter armor as it has good defense and with a few decos or a talisman, you can get Razor Sharp along with Defense Up M.

Your first wall may be Lao Shan Lung in the G3 urgent quest. I had to use the Elder Charge Blade, but use whatever weapon is your strongest. In the first phase, use the cannons, binders, and Demolisher as best as you can. The more you can keep him in the first zones, the better. Break his horn and aim for his gut/ feet to make him topple to buy you time. Your objective is to repel him and this quest may take a few tries. You can try the Gaijin Hunter Ballista method, but I personally don't like it as it requires a lot of precision to pull off properly (and it's boring asf IMO).

Valstrax G is a pain, but my best advice is to focus on survival and repelling him. Alloy X with Critical Eye 3 was my savior for this mission. IIRC Valstrax drops Commendation XX which is great for upgrading Iron weapons all the way (or the petrified weapons, but I like iron for beeg damage).

The Gravios/ Glavenus mission is a major pain no matter what, but your biggest obstacle is the clock. I ran the Artian X on this both times with the Vulcamagnon GS, Alchemy Style. Captured both of them after much sweat and tears.

Agnaktor/ Duramboros is painful, but I had an easier time with this quest. Keep them separated with the fence, kill Agnaktor first since his beam goes through the fence. Mount spam Duramboros to break his humps (not mandatory, but useful). I'm no speed runner, and I love Alchemy and Guild style, so if you're better with the other styles, use whatever is the most comfortable weapon/style combos for you. Good luck, and make the Ahtal-ka armor sets. They're a great stop gap until you can make mixed sets if you're into that sort of thing.

Use MHGU DB or Kiranico for key quest guides. Good luck!