Well, Ive played the Advance wars series from the first game and i have a couple stratgies that i find almost always help me win online/or offline with friends.
1. Mass rockets.(if there are no planes)
doesnt matter how many tanks or crap they have a wall of 10-15 rockets will stop them allowing you to mass war tanks or other strong units.
2. Never attack
when online ALWAYS play on the defensive unless your sure you can wipe them out in a few turns.
3.surround all squares around your indirect attack units.
you should do this anyways but make sure none of your indirect units are exposed to direct attack.
4. cruisers and battleships
Your cruisers can make a block infront of your battleships and your safe from subs and air allowing for your bships to attack.
when you have a good sized army just send out infantry for fun.
6. Tricking your opponent
This one is fun but only works in fog of war.
make a second army about the size 5-10 strong units.
send them in on the top or bottom (the opposite side of where your main force is)
and rush the enemy base.
If your enemy falls into this ( most of the time they do )
They will think your attacking from there but now you move your main force in and you should meet about 30-40% less resistance.
I have more but i think i want it kept to myself lol. :D:D:D:D