One of my favourite strategies in AoE II was to get a bunch of siege onagers and scorpions, group them with heavy ranged units like chu ko nu or hand cannoners, and throw in some heavy cavalry. Put them all into one group, select the box formation and set them to the Hold Position stance.
This creates a big movable artillery unit. The Knights surround the lot, but don't actively pursue enemies. Their job is to act as a meat shield to prevent melee units from getting too close to the ranged units. The hand cannoners and scorpions surround the onagers, and are very adept at silencing enemies at close range. Finally the Onagers in the centre are used to just pulverize any units at range that are too slow to get out of the way. You tell the whole lot to move to a spot, and you keep your finger over the stop key. as soon as you see an enemy, you just stop and let the onagers automatically target and pulverize them.
This tactic is dynamite against infantry, cavalry and short range archers. If you're playing Koreans, your onagers can be upgraded to have 8+4 range, enabling you to knock out enemy onagers, bombard cannons and longbowmen with ease. For other threats like monks and stuff, i also use another group of cavalry archers to take them out. This tactic is probably weakest against buildings like castles and bombard towers, but that's what trebuchets are for.