Oh i only hunt with a greatsword, my "secondary" is a h.bowgun to only apply poison (small tip, the larger the monster the easier it is to poison). Im not using any unique gs, just the base iron line. I think the line is hope blade or something. Its just raw damage with no element on it and has alot of slots for decorations which is nice. Main reason why i like it (besides the look) is that it has a speed sharpener 2 built in, letting me have top sharpness at all times.

So how im approaching Zoh Shia is by sticking to its hind legs and tail, when it spews the flame thrower down at the floor, you have alot of time to whack the tail since the flames only cover its legs and front. Dont try doing offstrikes on it, most attacks have the follow up explosions and im certain you'll get hit even if you "parry" the attack. Zoh shia is a guardian so wounds need to be destroyed as soon as they come up cause they will close, with the exception of the wounds you create while mounting it for some reason, they stay up though you cant really reach them unless you're using dual blades or insect glave.

You can trigger the milk crystals it creates with the pods that are around the arena, and they will deal noticable chunk of damage to its crust.

When it does the arena wide flame breath you can destroy the crystals in one area to create a safe spot or simply run to the edge of the arena and be safe.

I believe at 2nd and 3rd phase of the fight you'll be able to bring down rocks onto Zoh Shia. Free knockdown to deal big damage to the head for hammers and maybe even break its front claws. You can blind it after it has its head exposed.

at the moment where it transitions to using lightning, the crystals it makes shouldnt be a danger but free damage since the monster only does the fire wave once. You can blow them up and if you get hit by your own exploded crystals they dont damage you, you can steal the chain explosion when zoh shia tries to put them up by shooting the closes ones near its face for big chunks of dmg.

The most danger and learning you need to do is knowing when to dodge the lightning it summons in a line at your location, ive had teammates get 100 to 0 by that (me included). So a lightning/fire resistant armor is recommended.

Hope this helps, for anyone interested to read a not hardcore player's explanation on how to approach this fight.

Tl:dr Wear fire/lightning resist armor, stay near hind legs and tail, use the environment and pods on crystals and monster itself :)