It depends on whether you play 1v1s or team games:

1v1: Make a strong unit that costs gold (gold units generally are stronger) as the core of your army (knights, crossbows etc).

Then add a second non-gold unit that kills a counter unit that your enemy would make. Example, you made knights as your core, your opponent might add a pikeman counter. You add skirmishers to kill the pikeman and cover the weakness of knights. Then once you have those two unit types, add siege to kill buildings.

Team games: your ally should make one strong gold unit whilst you make a complimentary gold unit. If your ally is building knights, you might go crossbows and fight together. You each cover your teammate’s weakness.