Mainly inspired by the Harry Turtledove novels of lizard men invading the planet in the middle of WW2. Also by HG Wells and large numbers of 1950's B-pictures, in which the army always took on the monsters and flying saucers. They could also be used against traditional fantasy armies, for example in Kim Newman's short story 'D&D'.
Alien Army:
Stronghold: Weirdly glowing Alien mother ship or mysterious force field.
Airboat General (in large flying saucer) @ 3 AP - 1
Airboat (large flying saucers) @ 3 AP - 2
Artillery (Death ray) @ 3 AP - 1
Flyers (small rocket ships or saucers) @ 2 AP - 6
Alternatives: Dragon (undersea creature raised by alien technology) @ 4 AP, Beasts (alien animals or aliens without suits) @ 2 AP, Behemoth (giant crawling alien tank or tripod walker) @ 4 AP, Magician (Alien scientist with scantily dressed human female captive) @ 4 AP, Shooters (space suited infantry with lasers) @ 2 AP, Hordes (mind controlled human puppets) @ 1 AP.
WW2 Humans:
Stronghold: Ruined earth city or large bunker fortification.
Hero General (Patton, Rommel etc in armoured command vehicle) @ 4 AP - 1
Hero (Otto Skorzeny, Audie Murphy etc..) @ 4 AP - 1
Knights (Heavy tanks) @ 2AP - 2
Flyers (fighter aircraft, Stukas etc..) @ 2 AP - 1
Artillery @ 3 AP - 1
Riders (Lt tanks/armoured cars) @ 2 AP - 1
Shooters (heavy infantry with mortars) @ 2 AP - 2
Lurkers (Partisans/ guerrilla fighters) @ 1 AP - 1
Alternatives: Airboats (Lancasters, B-29's, etc..) @ 3 AP, Magicians (mad boffin in white coat with Bakelite equipment and Van der Graaf generator) @ 4 AP, Warband (Japanese or Red Army penal infantry) @ 3 AP, Sneakers (special forces troops) @ 3 AP, Spears (poor infantry) @ 2 AP, Hordes (Red Army or Chinese human wave infantry) @ 1 AP.
Alan notes: The human mad scientist should be depicted with a beautiful daughter if we're doing this properly.