What tactics should I use with Dungeon against Stronghold in online play?

I would STRONGLY recommend that you stop playing as a Pawn. And don't play as a Knight either. Ever. Moving on. If you do play as a Pawn, get rid of the habit of building a guild, market, and forge, as well as a town hall with a prefecture, in the first week. Instead, you should build as many troop levels and a castle as possible. Buy heroes, equip them with fast units, and send them out for scouting and resource gathering. Destroy necropolises. And also other resource storage locations (within reason, of course). In the second week, don't forget to build dragons and a castle (if you don't have one yet for some reason). And one more thing: capture external dwellings after thinking it through! Remember - if you have an external dragon dwelling on your territory and you haven't captured anything else, building dragons in your castle, even in the third week, will most likely guarantee victory.