Daisuke is a NPC you will eventually rescue from the Gungeon in one of the prison rooms. He will hang out in the right door next to the gungeon entrance. For 6 credits you can enable challenge mode, which adds a special modifier to every room to make the game much harder. The list of possible challenge conditions is extensive, and can be found on the official wiki, but usually it just means that there will be extra hazards to avoid or certain enemies you need to kill first or last.
I find that this is easiest to do in a rainbow run activated from another NPC you save who's in the same room as Daisuke. Rainbow mode give you a choice of items at the beginning of every floor, with some of the best items in the game being very common. The downside is you get no other chests or items from any other source. I just think that being able to have choice over what loot you get makes a big difference. This also lessens to need to fully explore every floor, because you can't get many other useful items from exploration besides just money to buy health and ammo.