If you play as Scotland, here's my preferred army tactic: Sticky Pikes
Scotland has the best selection of pikemen in the early game, and once they get maxed out, their late era infantry are unstoppable.
Scottish Infantry aren't the absolute best in terms of performance, but my god, they make up for it by being cheaper. Highland Nobles are like a cheaper version of German Zweihanders, and they will make mincemeat of people they attack.
So here is the Strategy.
Get a row of Pikemen, preferably Noble Pikemen or Highland Pikemen.
Line them up and the field and place them in the Spear Wall formation with defense activated. Start moving them forward slowly until the AI commits to a charge.
Behind the Pike Wall, have a shit ton of Highland Nobles and mercenary crossbowmen. Scots don't get good archery units, so pick these guys up whenever you can.
Once the enemy commits to an attack, halt your Pikemen and let them sett up. Their pikes will keep the enemy at a distance, but they will still be locked in combat. Once they've locked in, send the Highland Nobles through the Pikemen and charge them. Watch the enemy start to crumble.
As the routs start, wait for a moment until a large portion of them break off, then send whatever cavalry you own around the flanks and hit them as they flee. This will kill a good chunk of the enemy, and free up your pikes and longswords to finish the rest of the enemy. Wheel your cavalry around and hit the remainder from behind and watch you get a heroic victory.