During gameplay hold Tab PageUp simultaneously. Then let go and press the 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.
Hex Cheat Mission Select
goto offset 00000358
change that byte to 32 hexdecimal
save & exit.
You can now choose any mission you want.