Can you just look and tell us your ability scores and weapon? It's hard to help without the details.
For instance I would assume you are using a bow as a ranger, however if you're using a str based weapon with a low strength score that would make everything harder .
It is possible you're just getting low rolls, but your percentages should generally be higher than 50% for the Imps. They should also only take 2 hits max to kill.
I will admit that first fight on the beach with the intellect devourers has been a bit more dicey for me on some playthroughs. Don't remember many TPKs but one character has dropped on several occasions.
