Don Colton's Risk Dice Roller

(Risk is a trademark of Parker Bros / Hasbro for their board game.)
Wikipedia Article, Attacking

How many attacking armies
How many defending armies
How many skirmishes (max)

Skirmish 1   1 3 4   4 4 attacker loses X, defender loses X
Skirmish 2   1 1 2   4 1 attacker loses X, defender loses X
Skirmish 3   4 4 2   5 1 attacker loses X, defender loses X
Skirmish 4   6 3 2   5 1 attacker loses X, defender loses X