1. Best way is to attack coast cities first. If they have a stack say 3 inf/1 rcv as soon as you hit them once with ship AI sends everything besides 1 inf to sea. Just keep shelling the 1 inf in city the other escaping units will run into ship and basically sink themselves.
2. If run into stack away from coast dont charge at it take prov by it and back up and/or hit with artillary. once hit 1 unit will break away and chase after artillary so just keep backing up and easy to get that unit.
If unit breaks towards you is just a tank/rcv etc just dofge it and let it sit there.
3. Often if you create a border on an AI and wai a bit it will start breaking off any armor stacks into single units and send them into your territory. I leave them till board they cant do any damage.
4. Countries that are AI for a while basically break off all into single units and cover as many 1 unit per prov as possible. so I just send 4/5 stack of strike fighters to take out units in path of my inf. but if you just go in blind then end up speed bumbing each single unit.
the radar change makes this a little more tricky as will scout out area to see all the pprov with blimps and only send inf without aircover where blips arent.
And in general just be patient. if have artillary ; ships; planes just chip at them. never lose troops against AI
You said you know how to attack an AI country right? Could you teach me :D?