Battle auto selection

Best method I've seen recommended is to place all Blisseys in gyms and let them remain fainted when they come back.

I put all the most useless for attacking mons in gyms (read blissey). When they comeback I leave them dead until I'm ready to place them in a gym. That way they can't be autoselected.

I do think autoselect is somewhat based on defense/stamina and typing (resistances). Since for Lugia it always selects snorlax, lapras, and cloyster. When I was soloing a Machamp with C/DP my 7&8th man up was Xatu (useful) since both Psychic and flying are resistant to fighting. When I was soloing a BP/HS Machamp, it selected blissey (useless but high defense) and gyarados (passable) since water is resistant to steel.

Also, per GamePress, autoselect might consider the boss's move set. They write:

"You can 'tell' what charge move Lugia might have through the game’s auto-select choices. If you have a powerful Tyranitar and it isn’t auto-selected, you’re likely up against a Hydro Pump Lugia."

Still doesn't explain why it thinks Blissey is a good choice against Machamp, ....

I put Blissey and Chansey in gyms. Also don't heal Rhydons over 50% of their health bar. The game seems to love these too much