I remember once making a pretty interesting challenge. Iāve forgotten the exact details, but essentially it was fighting a boss with as few items as possible; not as few stacks, as few items, meaning 10 Wooden Arrows counts as ten items, not one. Life Crystals, Potions (buff, mana and heal), Mana Crystals, Armour, Mounts, buff furniture, Accessories, etc were also counted. More difficult modes gave you a multiplier of some sort (as in they made each item count as less items on harder modes, again, I canāt remember the precise numbers).
The rules are as follows: arenas have to simply be Platforms (unless using item slots for buff furniture), no hoiks or endless ammo items are permitted, and gear used must be obtainable by the point you would be fighting that boss in a by-the-book boss progression playthrough (eoc, eow/boc, qb, skeletron, deerclops, wof, qs, mechs, plantera, golem, df, eol, cultist, ml).
Note: Event loot is unlocked as soon as it becomes available to acquire.
Since there are few set-in-stone restrictions, thereās a competitive element too: you could easily implement a ranking system of sorts where those who used less items (or worse / lower-tier items, if multiple people used the same number of items) would be ranked higher.
My favourite bosses to try this with were Fishron and the Twins. It was interesting to see just how few items you would have to use for each boss, and to see other peoplesā solutions as well. Probably the best general tip that can be given is to use Gelatinous Pillion (if fighting a boss where itās available). Summoner and Melee seem to be the most viable classes, followed up by Ranger with a few no-ammo-used weapons such as Toxikarp and Piranha Gun.
The rules were revised a few times because I missed infinite ammo items on my initial post (theyāre utterly broken in this challenge) and because my wording didnāt quite cover what ānormalā boss progression was. I dunno if Iāve missed a few more items or tactics that defeat the point of the challenge, but Iāve been about as thorough as I can here.