Level by level comments:

Level 1(by Mars608)

Using MESHUGGAH's way save 5 frames . Thanks to him .

Level 2(by Mars608, aiqiyou, zyr2288)

Remove all the lags in the fourth room , better use of S gun , faster boss fight , 24frames saved .

Level 3(by Mars608)

Change the route a little . Do more jump glitches . Lowing down some jumps by pressing left or right . Get S and R for 2P . Get B and use it to kill a claw of the boss , for fun . Faster boss fight . 134 frames saved .

Level 4(by aiqiyou)

zyr2288 suggested using double S in this stage . aiqiyou made full use of double S . It's really hard to control it. 100 frames saved .

Level 5(by aiqiyou)

Let 2P die before the tank and use invulnerability time go throuth the tank , 2P cost last life before the second tank and keep game over to stage 6 . Because in double mode , P1 x pos is 168 . But 128 in single mode . So 1P can meet boss 40 frames earlier . 34 frames saved . fcxiaopengyou helped to test the route .

Level 6(by aiqiyou)

2p appears after boss appears and die before boss dies . 12 frames saved .

Level 7(by aiqiyou)

The same as level 6 . 10 frames saved .

Level 8(by aiqiyou)

2P get S in this stage and keep alive . 1 frame saved .