I no how you feel friend, I used to have this problem like 3 years ago when i first started playing monster hunter.
For me all 3 normal and g rank fatalis are really easy to slay and dodge.
If you find the fatalis easy to dodge but hard to take the health off then here is a way of completelly ripping the fatalis apart. read all of this, dont just read a bit and then do it and move on to the next bit, it isnt in any order.
get the heroics cat ability from your chefs, and make sure that there specialities ar bran or/and milk with heroics. Heroics makes your attack go up by an extra 50 % when you have really really really low health.
also (if you can or want to) get potential +15 this will make your attack go up by another extra 30 %
so your attack will be an overpowering 80%, remember both abilities will require you to have very low health to take effect so bring toadstools and barrel bombs to take your health down.
power charm will help and if you have a lou shan lung claw mix it with the powercharm and it will become a powertalon and then buy another powercharm
make mega demon drugs aswell they boost up your attack loads, only use one as they last forever as long as you dont die if you do just use another one, and also bring power seeds because all these things stack up your attack. also if you are using a blade then i suggest getting the aristan ability as it will make your sharpness purple depending on your sword. get the sword draw ability, sword draw makes it so when you get out your sword with an attack, ( like running and pressing triangle) it makes it loads more powerfull.
If you want the potential +15, sword draw and aristan armor then get the full set of white monoblos armor (also known as modievil or something similar to that armor) and jewel it with decorations, (this is what formula armor I used). remember also that dragon element is the same as raw attack to fatalis.
So if you are sure that you will not be hit and you are willing to get the armor and jewels with a good weapon. you will completely anniolate all of the fatalis including the g rank white fatalis, trust me i used this stratagy and I have allready killed 3 g rank white fatalis on my own. I calculated my over all attack at 2800 and something lol
here is a website which will gives you weopon, armor, jewel, abilities, and skill lists and pretty much anything you need to no about obtainable stuff in monster hunter.
Hope this helps and good luck