you start on a completely new safe file and the timer starts when u open this safe file for the 1st time. Then you try to get your 1st checkmark as fast as possible which i believe would have to be either mom's heart with any charecter on ur 2nd run or greed with isaac on ur 1st run.

just, why ?

Sounds fun? + i feel like there could be a lot of strategies involved into it

"i feel like there could be a lot of strategies involved into it"

The first completion mark would be Mom's heart. So the only strategy would be to reset for a good starter and find a black market on D2 to skip mom and go fight Mom's heart.

Congratz, this is a worse 1 char category