InsaneCommander

Oh, you don't wanna bring Megumin inside that dungeon. Her clone would kill everyone with a single "Expurosion".

Last boss:

Momodora: Moonlit Farewell. Another good metroidvania, it looks just like the previous one and it's beautiful. It has no complicated or annoying mechanics and I enjoyed it a lot. However, I do have many criticisms: the map is more linear than in the previous game, basically one straight path with some ramification areas (although each area has many short ramifications). Most bosses can be damaged at any moment and the battles become a little boring, with victory depending more on pressing buttons quickly than dodging and trying to find a weak moment/spot. The story is quite simple and the companions could be more creative, but instead are just skin variations of three types.

It's an improvement over previous systems:

Scientists Show Off Levitating Material that doesn't rely on any external power sources

It's been quite some time since I watched this, so I was confused in some scenes, but I still didn't want to watch the recap. It's interesting to compare this with Overlord. In both cases you have characters capable of extremely evil things, but in Tensei Slime they only do it to people who do terrible things to them. If I had to be an isekai character, it would definitely be Rimuru. The guy can recreate anything he wants, including food, mangas, healing potions...

As @melkathi said, this is Transiruby. It's a nice metroidvania, although a simple and easy one. And no, it's not available on GOG. The bike form was an interesting idea, but they should've made large horizontal areas to truly make use of it.

It took me a while to finish Rebel Transmute. The game is good, but at some point it started to be really annoying, with bosses far from save points and some game mechanics that were a little too hard to reproduce. Eventually, I simply decided to enable infinite healing on the accessibility options and rush to the end to see how the story ended. It was nothing that interesting, just a waste of time. Transiruby. A simple and straightforward game, but compared to Rebel Transmute this was great. Too easy, but at least it kept my interest all along and never bothered me. The graphics are quite simple but I liked it a lot.