I use to make anachronistic battles, which may seem unfair, but not so much if we take into account rifles and pistols are not better than swords or even sticks as they do not shoot. Catapults do not need to be any worst than cannons.
In my Playmobil world, Roman armies can fight Medieval knights, and pirates can fight US troops. In my world, time has passed at different rates in different places (for example, Rome, Nuremberg, and the US West).
Romans can compensate their worst artillery by the use of their big shields, which make more difficult to make them fall by shooting them with artillery. In my particular case, they have the advantage that the lithobolos torsion catapult I made is the strongest artillery piece I have, both in reach and impact strenght (however, I just made one of them). They have also assault towers which enable them to be protected even when advancing.
Some tips:
-Cavalry is the force most likely to get engaged in body- to body combat, try to give them the regiments with more clickies with hands that grasp more firmly.
-If you do not have Medieval knight with lances (lances with guards), avoid cavalry with that kind of lances, and direct to any other target. They had advantage over other types of cavalry, as they can send them to the ground. This has some logic: a knight with lance can reach a cavalryman with a saber (for example) before the saber guy has its opponent at the reach of the saber.
-This greater value of the heavy Medieval cavalry implies we should try to shoot they down them preferentially (catapults are better for this). They can not only drop cavalry, they can also engage in body- to body combat also.
-Infantry needs more time to reach the enemy, and so more exposition time to artillery. Try to hide them behind cavalry from the enemy line of shooting, to make their advance easier. Hide them and try not to make them advance if the army does not have much cavalry.
-Try to not to advance if you have better reach than the other army. Let's the other support the casualties of the advance.
-If the hideouts of the other army are too difficult to shoot, you better forget the gun power and advance. You can move your cannons by losing a turn of shooting for the moved cannon (as in other rules), but you need to dedicate a horse at least to move the cannon (thus limiting the numbers of your cavalry).
-all Playmobil cannons may be able to get horsemen down (if sufficiently close) by using larger home-made bullets. Try not to waste them against cavalry. However, you need to shoot the horseman from the side (in a disposition like that of Balaclava, you can hardly get a horseman down if it is headed towards the cannon). So, try to head the cavalry towards the cannons. An so, try to put the artillery aiming trajectories at 90º angles (each horse can not be headed at both cannons at the same time).
-Hide the defenders in as much as possible, but the cannons may retain a not so-hide position so as to still being capable of shooting in the position it is. this will tend to make the defender artilliers not- so hided and vulnerable to attack. On the other hand, you can hide your defenders more, sacrificing their firing capacity, but it is senseless: it only means to retard the coming man- to man combat.