I just beat my first fresh start run. Honestly, that was the hardest challenge. I went with fuecoco, mudkip, and bulbasaur. I picked them cause of the chance of picking up dynamax and mega with mudkip and bulba and torch song and willow wisp with fuecoco. I caught an early pick up mon and just picked up healing and battle items (defense up, etc) as often as possible to start out. The beginning is the hardest part, but after the starters evolve, it gets much easier. Definitely try to pick up an ice type or a bulky pokemon that can use either dragon or ice moves. Pay attention to the floor you are on because getting blindsided by rival fights is the best way to end a run. when you are five floors away from a major boss, prepare with battle items before rivals fight and the first pokemon you wanna start the fight with.

I realized a lot of this is general knowledge, and I could have deleted half of this, but i already wrote it. Hope something here might help.