Thanks for playing!
Multiplier is a bit obscure I give you that, I may add a section to the page but it's already long enough as it is ahaha.
Since there's not really a combo system (you only have 2 attacks really, the sword and the crates) the multiplier works like this:
Basically, the game keeps track of when you hit an enemy, killed an enemy and your movement speed.
Every action gives you a weighted score, and the result is normalized into steps (1x, 2x, 3x, 4x, 5x).
The multiplier also drains on a timer AND even more if you're moving slower than X speed.
You also lose a bit if you attack and miss (either sword miss or crate miss).
And of course, if you get hit it resets to 1x.
So really to keep a high multiplier you need to run around like a maniac and keep hitting enemies as much as you can.