Haven't played MTW II enough, and not for a while, and I suck at online =/
The best advice I can give you is to just keep your army tight, and make it more infantry based when going against factions that are calvary centric. Calvary, once they are bogged down, will lose to infantry.
Make the infantry getting hit thick, have a back up unit behind it, and then surround that calvary unit with your spare infantry.
Take the longbow men off auto fire, and do simultaneous firing against calvary to weaken them.
Also, remember that your Longbow men have the spikes, you can use those too, although I've never used them before.
If what you say is true though, you're at a bit of a disadvantage, since you'll be playing a passive and defensive strategy, and that'll get broken eventually.
