You can beat the last boss guranteed, thank me later(assuming you have boost, if not, try getting it):
Go to bottom left, beat the powerful, get his Nidhogg attack (Enemy's Max health -1).
Now, go bottom right to beat the wise in two hits and get his 1M health. (By using boost and your new attack)
Next, go top left to beat the damned and use boost. When he dies, take ANYTHING besides Explosion. (Fenrir is coolest*)
You'll be left with an extra boost and can kill the next enemy you fight guranteed! Just make sure not to get hit by anything on your way to the Almighty and you got yourself a one hit kill!
*About Fenrir: It's basically invincibility. You can keep using it as long as the enemy would do anything dangerous.