If your early game then yeah thats going to be rough but mid to late game your running around with 300+ then add ur allies and vassels that make it cake. Best to avoid it. With a decent army you could beat them with 300. Play it safe.

If there's a feast or tournament being held at a nearby city, you might be able to lead the army over. If all goes as planned, all the armies at the feast will leave the feast to attack with 1500+. If a faction is on campaign, you can do the same thing. It takes some luck and a lot of training around, but it works. You might look for a choke point on the map where the army has to path through. Train the army around that choke point until some friendlies decide they don't want it around anymore. It's a good way to get good loot and troops early int he game.

Hit and Run. You need a fast horse and good armor, bow and 3 slots of arrows and a lot of time. Plus you need to set the map size to its larges size in camp/PoP options/battlefield size. Drag the army to some flat land then attack it. Your going to need flat land because while your riding around shooting you don't want any trees or boulders to get stuck on or hills to slow you down while your being chased.

TIP: When you first enter into battle "charge the enemy" if you don't like the map just press tab and retreat then select "charge the enemy" again and you get another map. You get a choice of 3 maps per battle.

When you start a battle send troops to the opposite side of the map. Your going to have to keep an eye on them and moving when the enemy gets too close to them just press back/delete key to see friendly and enemy locations. Then charge by your self. Just keep shooting into the larges group of troops while dodging enemy cavalry, troops, and map obstacles. I usually charge with the army on my left side while shooting into the on foot troops. Usually half the cavalry will chase you while the other half goes after your troops. On foot troops will go after your army, you can actually come up behind them and shoot them from behind or just circle around them and keep shooting. You can also let your companions tangle with the cavalry that catch up to them. Your decision, if your companions don't have good equipment and skills they'll get steamrolled.

When you run out of arrows just retreat and and re-enter battle you loose moral but if you killed enough troops you gain moral too and break even. If you get knocked out or have very little hp left just leave the battle for the nearest town and rest up.

When the enemy army is a manageable size, when the game gives me more than just my companions vs enemy army Ill go finish the battle.

Again this take a really long time.