Use Axel with his Laser-Beam-Weapon, its no problem to come to level 
3 or 4 without losing a credit

These tricks all require you to watch the opening demo entirely. When the 
Title Screen comes up (with the title and TPUSH STARTU), press and hold 
L1, L2, R1 & R2 buttons on controller 1, then press SELECT. TPUSH STARTU 
will change to TSECRET CODEU with two letters behind it. The letter at the left 
is changed by pressing up or down. The letter at the right is changed by using 
the Triangle and X buttons.
Stage select:
MA Stage 2
UT Stage 3
RH Stage 4
MK Stage 5
HT Stage 6
Special codes:
CM Makes character small (does not alter hit check size)
QB Makes character huge (does not alter hit check size)
MV Puts up smaller windows on the screen by pressing any button on 
controller 2Us face.
YI Start with nine bombs
TY Axel/Ruka turn into one-hit wonders
SS Weapon power-up time starts with 999 seconds
Debug Mode:
Set secret code to MA, press SELECT, then change code to SV and press 
START. The following commands must be entered using controller 2. 
Up Boosts weapon power to max (10-second duration)
Down Voice mode on/off
Left Gunlock type change
Right Skips Area (disengages invincibility)
Triangle Invincibility on/off (falling will cause damage)
Circle Increases number of bombs
Square Switches selected weapon type from Axel Ruka.
X Increases weapon power up time in 30-second increments.