Here are all the level codes for all skill levels for Commandos 2. In order to use them goto single player, then pick your character id name and then enter them under keyed mission and hit enter.

Level 1

Normal Difficulty, XHGDR

Hard Difficulty, PLKUM

VERY Hard Difficulty, PVTSL

Level 2

Normal Difficulty, WKUC4

Hard Difficulty, JE5SH

VERY Hard Difficulty, SKDJF

Level 3

Normal Difficulty, YSM51

Hard Difficulty, DFY3B

VERY Hard Difficulty, 3DYNG

Level 4

Normal Difficulty, B7D8F

Hard Difficulty, K9D3H

VERY Hard Difficulty, 9BG3S

Level 5

Normal Difficulty, 3GHSL

Hard Difficulty, NMWQ9

VERY Hard Difficulty, KJWJK

Level 6

Normal Difficulty, AZLM1

Hard Difficulty, 16G3L

VERY Hard Difficulty, E2J7H

Level 7

Normal Difficulty, JAHSG

Hard Difficulty, WL3CZ

VERY Hard Difficulty, ZX78Y

Level 8

Normal Difficulty, UN63A

Hard Difficulty, LPQ6T

VERY Hard Difficulty, TRIB4

Level 9

Normal Difficulty, VAZ2P

Hard Difficulty, SRCM8

VERY Hard Difficulty, TRD78

Level 10

Normal Difficulty, 9TT5W

Hard Difficulty, PAEN8

VERY Hard Difficulty, 1LPQD