After you load the artillery, get on a horse to tell the line to retreat, you will have to rescue stranded soldiers. After this you will have control of 2 howitzers and a near endless wave of Russian soldiers will be coming at your front line. Just take direct control (CTRL) to fire on the areas where there are lots of Russians together. If you run out of ammo use Franz to get some from the wooden crates. After a few minutes the horde will thin out and itll say "the Russians are retreating" and the achievement will pop.