easier way to score 20000+

[Sombroff]

if you find that stage 1 boss is hard, just do a game over here just before to kill him,

then you press Continue, your score will be reset to 0 bust just afer kill this boss and finish stage 1 you have some bonus points,

and finish stage 2 is easier than finish stage 1

in this way you will have more than 20000 even before start stage 3

[Fallen_Chaos_]

The easiest way I've seen is to have a second controller and just beat up on player 2, then when they die just press continue on their controller rinse and repeat until you reach your score.