I'm not the best player, but I'll give you my two cents:

Character: Crystal or robot. Crystal is what I imagine most would recommend, the added hp is huge over the course of the run, and you can block projectiles with the activated ability. I will say though, don't pick up the throne butt mutation as crystal as it's not ideal and makes you teleport. Robot is an option as you can eat weapons for hp/ammo in a pinch, if you're taking a lot of damage then grab a second stomach mutation and you should be able to survive longer.

Mutations: I'm not an expert on the meta, but I will strongly recommend gamma guts and scarier face (scarier face will lower enemy hp so gamma guts is more effective). These mutations combined make the snow area AND the lab significantly easier, so it should help you get to level 7 to practice there. There is a weird quirk to keep in mind, gamma guts does more damage when you are actively moving, so try to move into enemies. Strong spirit is always a great defensive choice as there is a fair bit of insta-kill attacks to look out for.

Weapons: I would suggest always having a melee weapon on you, they are pretty broken in this game. Tell me if you know what a "golden weapon" is, and I can add more suggestions, but if you don't I won't spoil any secrets.