How do you even get to boss rush with tainted cain?

[VoltX63]

It's one of the slowest characters and you can't even build whatever you want you just wait to get garbage items

[kone]

Have to be lucky with crafts.
Low quality, but strong items with very basic materials you can find on Basement 1:
Glass Cannon, The Candle, Red Candle, Shoop Da Whoop, Stitches, Sharp Straw, Larynx, Almond Milk, Sacrificial Dagger, Fruit Cake, No. 2, Camo Undies, Lachryphagy, Tech 2, Purgatory, Red Stew

Might as well try out 'bad' items like Isaac's Heart or Bob's Brain because if Boss Rush is all you need then those items ARE fast for this character.

[☠ Oda Nobunaga ☠]

Mama mega

[Eprico]

https://platinumgod.co.uk/bag-of-crafting/index.html

Look at the fixed recipes. You can farm the fixed recipe spacebar items by crafting them inside secret rooms which guarantees that they generate at least 1 bone heart. You can use this to infinitely farm R keys and strong items. Then you can either farm until you get lucky and get mama mega or you can calculate the recipe using the website I linked above. Using this method you can get all the marks apart from ultra greeder in a single run in under 2 hours

[Kharn the Betrayer]

https://steamcommunity.com/sharedfiles/filedetails/?id=2488481628

[Well Met]

Reset until you get a quality 7+ drop from your first treasure room and if RNG is on your side you'll be able to craft something half decent which will carry you to boss rush. That did the trick for me when I got a golden bomb and a lucky penny from the first treasure room item. Look for a recipe which contains your drops + other drops which are trivial to get. It's an easy way to craft q4 items. For sure easier than going for Mama Mega and shorter than farming special hearts. This item + some garbage items along the way should be enough for Hush as well.