F17 CHALLENGE
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I've found a cheat to stay at 0 per cent damage. When you reach a tunnel
crash against the left wall and hold the joystick there until you reach
100 per cent damage.
You then hold down the fire button and your damage goes up to about 700 per
cent when it resets to 0 per cent. Make sure you are well inside the tunnel
or you will reach daylight half-way through the cheat.