Simulating an Army Battle

Using the Lanchester's square law, I need to model a situation where two armies, army A and army B, each have 1000 units of men. Army A may fire every 30 seconds, while Army B fires every 10 seconds. The units in army A can disable the opposing soldier in army B once every 10 shots, while the units in army B can disable a soldier in army A once every 20 shots. How would I go about plotting the number of soldiers in each army as a function of time for 30 minutes of the battle?