Video by garadas21:

Explanation:

If you are down to one heart remaining, your jump attacks are capable of doing more damage. So it's essential that you be down to one heart entering the boss battle.

Double jumps are used for this, so make sure to have the boots.

The boss is on a timer and will begin blocking and counterattacking after a fixed interval, so it's essential that you take him down before that time.

Make sure to allow the attack's full swing to progress before each jump and attack, so that the full damage from the swing comes through.