Looking for games like Warning Forever. They do not have to be shumps. I would like to fight bosses or enemies with the minimal of exploring.

I don't want to unlock a boss rush mode I want the game to be already setup like that.

Games that I've found that fill that itch

* Crimsonland
* Galaxy Champions TV
* Dungeon Souls

I feel like SHUMPs are my default games because I can share a controller with a friend and we keep it moving. It also does not require exploring and I'm not into exploring in games.

Maybe Cuphead will be for you. It is a run and gun platformer, and most of the stages are boss battles:
https://www.gog.com/game/cuphead
It is quite pricy, so you can wait for a sale.

Furi is the first thing that comes to mind, and probably the most immediately helpful answer.

also if you mentioned Warning Forever, I assume you're also familiar with Kenta Cho's rRootage since it's also freeware.

there's also Alien Soldier (besides emulation it's also available on steam, switch online expansion, and various sega collections), and the titular mode in Dodonpachi Daioujou Death Label is a boss rush available from the start (although you could probably count the number of people who cleared it on one hand). Ketsui Death Label isn't a port of the game due to the DS not being able to handle it but rather a series of progressively longer and more difficult boss rush modes culminating in the titular Death Label mode and Doom Label mode (where you fight progressively harder versions of the game's true last boss).

I second Cuphead. Virtually every stage is a boss with a very few platforming levels in between (i.e. there are only 7 "run 'n gun" stages in the whole game)

https://www.gog.com/game/furi

Never played it, but enjoyed watching it and people say it's really good if you like this kind of '1v1 duel' boss fights with unique mechanics each.

Just now noticed mrcrispy83 recommended Furi already, sorry. :P

I third Cuphead and tomorrow (actually today) release the DLC.

Cuphead rocks hard. It's worth every penny.