I had a lot of fun playing this! The Slime Keeper guy was very... intense. I love that he keeps piling up the slimes you collect on the counter before him.
The FP system was cool and I felt it had a lot of implications on the gameplay, like the fact that moving was much more expensive after attacking.
It was fun trying to figure out how to best combine the weapons you got to deal as much damage as possible. The order in which you used them was very important. I guess you want to use the ones with low FP first and then switch to the ones with higer FP cost.
It would have been nice being able to switch weapons with fewer clicks I guess, since it was something I found myself doing several times every turn in order to maximize the damage. That's just a nitpick though. I think you did a really nice job on the UI. There's even a mini map! And the hover effect when moving or attacking made it easy to figure out how to play the game.
Maybe highlighting the weapon icon when in attack mode and the character portrait when in movement mode would be a nice little addition to make it more intuitive. It took me a while to figure out how that worked.
There's a lot of depth to the gameplay and I liked how the enemies were smart enough to move out of the way so their buddies could surround you, it made the combat more strategic and meant that you had to kite them into choke points.
Great entry! And super cool that you did it all in vanilla html5