Metal Slug is described as 'With a long history tracing back to early stand-up arcade systems and NEO GEO console, Metal Slug (series) is the definitive bullet hell platformer'. There are five games similar to Metal Slug for a variety of platforms, including Windows, Steam, Xbox, Android Tablet and iPad. The best alternative is Cuphead. It's not free, so if you're looking for a free alternative, you could try Mr Autofire. Other Metal Slug like games are Guns, Gore & Cannoli (Paid), The Bug Butcher (Paid) and Rise & Shine (Paid).
Cuphead is a classic run and gun action game heavily focused on boss battles. Inspired by cartoons of the 1930s, the visuals and audio are painstakingly created with the same techniques of the era, i.e.
The Bug Butcher is an intense action shooter, wrapped in stylistic 2D art, and smeared with humor where timing and fierce reflexes are paramount! The core mechanics hold old ties from the classic arcade shooter “Super Pang” where you shoot vertically, but with modern day twists.
Rampage through levels and shoot the alien scum in the face. Or behind, your choice. Eventually you’ll get yours kicked. Tough. Sucks to be you. But! With the loot you can find, you can get bigger and badder weapons that’ll leave a string of ruined enemies in their wake!
Rise & Shine is a true "think and gun" that combines elements of arcade shooters, bullet hells, and puzzle platformers to create a new blend of strategy and viscera. Switch between Shine's add-ons to solve puzzles that redefine what bullets can do.