Lots of questions here. Like what kind of gear are you wearing? What weapons/shield are you using? Is all of it upgraded? What are the "best foods" you have right now that you are using?

For myself I am in the plains now and have T4 iron armor, a huntsman T4 bow and I'm using obsidian arrows. My food is Turnip Soup, Wolf Skewers and Eyescream.

Did you go to the plains before the mountains? The Mountain/snow biome is supposed to be explored and cleared before Plains. Nothing can come close to 2 shotting me in the plains.. I think you may have skipped a step somewhere.