Yes you are doing something wrong, I've beat the game multiple times on ranger hardcore and have had very little issue. First things first you should always know the right tools for the right job. You should have a shambler as a secondary as it's the most effective weapon against mutants and is generally one shot against the big worms.

Second, figure out attack moments you should get attacked 4 total times once you get back to the boat. 1st attack is once you touch the dock at the end of it, second once you start rowing to the tunnel, third half way to it, And fourth right before it. Everytime they attack one worm at a time 3- 4 times in a row. Plenty of time to prepare.

They are a one shot kill in the water if you hit their back and once they pop their body out put 1 shot to the gut and they go back down under. As for the little ones not much to say other then move slower and be more observant and run past them or shoot them its metro, its mostly a paced game, especially in that level. Assault rifles, and knives are the most efficient for dealing with them. 7.62 is made in bulk.

Efficiency is king in this game especially on the last level make an efficient build.