can someone help me improve my build?

mapping isnt hard but i cant really kill bosses https://pobb.in/Cy6CtyqXldEY

share your POB link

1) You have no crit chance....get rid of abyssus and get a better helm. It's doing far more harm than good.
2) Molten Burst (from weapon) is NOT a melee attack. Pulverise doesn't work. Increased Critical Strikes is mostly wasted through #1. Faster attacks doesn't work because it has a set cooldown. Vicious projectiles works, but is less effective because you are converting a lot of damage.
You should use: concentrated effect, returning projectiles, elemental focus, awakened ele damage with attacks, fire penetration, etc.
3) You simply need better gear everywhere.
4) Get rid of any "gain no charges during effect" flasks. That is basically never a good thing to have. Why use a jade flask on an armor character? Rotgut does not fit your build. You don't need 2 life flasks.
5) your best support gems should be on your cyclone, NOT the molten burst. They aren't doing the majority of your damage right now.
6) Test supports and see which gives you the most FIRE DAMAGE, not physical damage. With the exposure on your gloves, and the buffs from the tree your damage is going to be mostly fire.
7) You can't run two mark on hit setups........you only get 1 curse unless you have something that increases your curse limit. If you continue with crit, use assassin's mark. If you don't, you'd be better served with flammability than any other mark.
8) *Important: Your build is NOT an ignite build. The ignites you deal hardly have any damage. Get rid of ALL the ignite nodes you have on your tree (along with any fire damage over time nodes). You are an "on-hit" fire/physical damage dealer. You need to be buffing primarily your straight up fire damage, along with some attack damage, and then physical damage is third. Ignite chance and damage over time should be non-existent.

I'm assuming you are having survivability problems? 300k dps is enough to do a lot of content in the game upto end game bosses.

However 2k life and abyssus on a melee build with very little defensive layers and is your cold res under capped? I am surprised that is clearing maps without frequent deaths. Endurance charges on kill might be part of what is keeping you alive while mapping, but if they are not up while you are bossing, you are loosing one of your only forms of defense. Fortify as well. That is usefull, but is it proccing on the bosses with full uptime or dropping off?

Without looking at the build itself, I am not familiar with Ngamahu flame, but I would think you need more life pool. I don't think you have stun immunity on this build, so with that life pool is your cyclone getting frequently interrupted when you get hit? If so, you might want to take unwavering stance keystone, which you are fairly near, or warriors blood in your thread of hope, combined with more life nodes and life moods on your gear. The way leech works is based off your maximum life as well, so more life pool means more leech recovery per second, if you are struggling with your life recovery. Maybe this is all less relevant when using petrified blood, still seams like very low life pool though. If relying on flask to keep near max health, could be a further reason why bossing is tough if the flasks are running out.

The vast majority of builds run a guard skill as well. Only guessing, but I think for this you would probably want steelskin/automation, because of the low health pool.

Your tree looks a bit inefficient in places, but there may be bigger things to fix than that. Since you pathed to get some spell suppression, I would take the Lucky spell suppression mastery as it will basically double your spell suppression.

The most helpful thing for you right now is to get t2 or t1 life rolls on every piece of gear. The armor is hurting you the most: I know the -15 is maybe necessary but you are losing a ton of life with that low roll. Same with none on your gloves or boots. No matter what your defensive layers are, a low lifepool will mean instant death.

After that, on an armor 2h build, you are going to need +max resists if possible on your passive tree as well as endurance charge generation somewhere. The curse gives you "on kill" but you need another source vs. single targets. This could mean having to use enduring cry.

^Those two things are going to really be the bare minimum to push you further defensively.

You should really have 6 molten burst supports in your weapon. The weapon should have (don't need to be linked): fire penetration, concentrated effect, awakened ele damage, vicious projectiles, combustion, and then probably greater multiple projectiles.

You cannot use steelskin AND molten shell. Automate your molten shell with high level cwdt (CWDT - Molten Shell - more duration) OR alternatively keep it low level for higher uptime (CWDT lvl 1 - Molten Shell below req. lvl 38 - more duration).

Triggered skills, like molten burst, are NOT counted as being used: Hrimnor is not doing anything notable for you since cyclone is NOT a fire skill. Get a different helm with high life and armor and maybe other mods that seem nice to you.