How to continue Hard mode Dawn of the Infinite

I’ve done the first 3 bosses with a group, after whiping a few times, we stopped at the fourth boss. Now I would like to continue on the fourth boss, but I cannot figure out how to get it back on hardmode after entering. Is this possible to do or do you need to start all over again on the save of another player?

you cant btw on hard mode it gone this week moment u fail or death u fail ach

It’s not for the achievement, I want the heroic Stonescale trinket. If I continue on normal mode it’s champion gear