I mean if you go to the plains with iron (i'm still there as well) and expect to facetank everything you are indeed down on your luck. You can take on a single mosquito or a goblin but not multiple of anything, especially not melee. Don't bother with Lox', Berserker at all or those Tar thingies either, except for the latter you can lure things into them for the Tar (asfaik it's cosmetic building only), but by doing so expect to die and preferrably not close to a Tar pit because it makes tombstone runs hell. Anyway.. beside the Needles for the best arrows you don't even have business there in that biome except for crossing paths to other biomes and in which case it should be done sparingly. (Getting multiple mosquito's is almost always a death sentence). As for the goblins you can kill them rather easily same way as you do trolls.. just kite and shoot with your bow, same goes for a single mosquito. Also when crossing the Plains for whatever reason, always expect to die and have a spawn point closeby.

This is all from my experience so far in the game.

Edit: (TL/DR:) At our stage (iron) using Obsidian arrows is much less of a hassle lol and we have absolutely no business in Plains biome yet, except to cross it into other lands, which should be done with great care and only small distances, but using a boat to skip it would be even better.

P.S: Gather Cloudberries, as many as you can. They are the one thing that makes your deaths kinda worth it (at least at my point in the game lol). Better than many stamina foods you can make but require no cooking recipes and they grow in bulk.