On easy mode, boss has 250 HP and does one of two attacks: one-hand attack or two-hand attack. The one you need to watch out for the most is one-hand attack as it has longer reach and is slightly faster. Because of that, when dashing backwards you need to hold left for a moment longer if the boss is using one-hand attack so you don't get hit..

Air combos simply don't work on the boss. Doing backflip attack deals double damage (20 damage), as opposed to only 10 damage on Fire boss. However, the good combos that make use of backflip are risky and are only included in the harder strat.

Top-left part of the video are easy strats. They are only half a second slower than TAS strats! You can do them every time, RNG doesn't matter. What you gotta do is simple:

1. Charge, shoot fireball after connecting with charge, 3-hit combo, dash backwards without getting hit (dash slightly more if boss is doing one-hand attack) (184 HP)
2. Repeat step 1 (118 HP)
3. Repeat step 1 (53 HP)
4. Charge, do 3-hit combo (0 HP)
5. Quickly dash to the exit and turn around before reaching the closed doors, then dash backwards.

Top-right part are hard strats. They are over half a second faster than TAS strats. They require the first boss' attack to be two-hand attack. Otherwise, you won't have enough time to do the backflip combo. If boss does one-hand attack after you finish your first backflip combo, there's a high chance you'll get hit unless your execution was good. What you gotta do to kill the boss faster than TAS is:

1. Charge, shoot fireball after connecting with charge, 3-hit combo, dash backwards but not too far away (184 HP)
2. Immediately charge forwards, shoot fireball after connecting with charge, 3-hit combo, backflip (A+B+left), immediately after tail spin and hold left longer if boss is doing one-hand attack so you don't get hit (88 HP)
3. Charge, shoot fireball after connecting with charge, 3-hit combo, backflip and immediately tail spin (0 HP)
4. Quickly dash near the exit. You won't reach it before the cutscene, but you're still saving time overall thanks to faster boss kill.

Bottom video is what TAS does. My hard strat is faster than TAS by over half a second, plus it doesn't require you to waste over 30 seconds getting combo upgrade at the training dummy!

You can watch comparison of all 3 videos here:

You can also slightly modify the easy strats to include 3 more fireballs (one fireball each after 3-hit combos, before dashing backwards), which would mean you save about half a second at the end because you kill the boss with 2-hit combo instead of 3-hit combo. But adding a fireball between 3-hit combo and dashing backwards is risky, you need really good execution to dash before boss hits you. I didn't include this strat in the comparison. It would be about as fast as the hard strats.