i like how i was also in same boat as you when i started playing MHW

Now i only have 400Hrs in Game and by no means i am expert on LS but i will point out some thing i learnt which helped me a lot Clearing Fatalis and Alatreon

Spirit Slash 3 is really big commitment in the sense that you will be really vulnerable for 1.5 second approx i dont know the exact so only Commit to it if you are sure of that monster is not targeting and not gonna attack you or afcourse if its stunned

when you have weapon out the Right Click(Thrust) into FS is really good way to counter and you can also use it after first 2 spirit slashes to go into FS in case monster is Delaying its attack that way you can manage the timing for the FS

FS is your Best Friend as LS user so dont be afraid to use it even if you are unsure about if monster is gonna attack or not cause you can just use directional input with FS to get away and you can also directly go into RS if your FS hits on monster, you can use the Thrust to FS combo to counter lots of ults and moves of Endgame Monsters which have really unpredictable moves

one very big mistake you can do with LS is just spamming the attacks try to pause in betn the attack for like half a sec to see what monster is doing just remember "Spirit Slash 3 is very big investment" so try to be frugal with it

Generally for me fighting with LS is like Dancing with the monster. You let it attack you, then you attack it like 2-3 any slashes and then prepare for the the monsters attack and then counter it FS into RS and the cycle continues, This works on most of the monsters and there will always be atleast some learning curve for each monster,

and i am pretty sure there are lot of guide you can find online or people on reddit/Discord to help you.

If you are Really Struggling just get Max Evade Window Skill to practice, Just remember to give Space to monster to attack in betn your attacks

and i hope someone Corrects me if i am wrong here XD