During gameplay, hold down TAB and hit the page-up key simultaneously. Then let go and press the letter P. At the bottom of the screen it will say "cheat enabled." This will make you indestructible. This won't help on missions where you have to keep other things, like the War Drum, from blowing up, but it will help in those nightmarish combat situations.

Or you may try this alternate hex cheat:

HexEdit dl.cfg, goto offset 00000358, change that
byte to 32 hexdecimal, save & exit.... You can
now choose any mission you want.