The very first and last frame of a screen gives you 2px/f movement speed (so pausing or other tricks won't work)
You can't move while attacking forward
You get invulnerability for 128 frames once damaged and you can take up to 5 hits (10hp) before dying
Optimal boss fight
First boss: 255 hp
Fastest way to beat bosses is running towards them and positioning your sword hitbox inside boss hitbox to deal 2 dmg per 2 frames. You are invulnerable as long as you are in air and attacking, therefore you can't be inside in boss hitbox if you can't jump AND dmg at the same time. If done correctly, you are supposed to deal the next damage with 1 frame delay (3rd frame after the last hit).
Because of the hitbox sizes and the movement speed of specific enemies (and you), sometimes you can't avoid them without losing life. Thanksfully, completing a screen refills your health (but not before a boss battle).
no, 9 frames
Outcome depends on time and input. Bad luck is 121 frames shorter than good luck which gives you 2000 gold (instead of 0 gold)