Then the slowest guys next to ranged attackers so if flying or otherwise fast enemy unit go to attack the ranged attacker, you can kill it with the slower guy. Especially rampart's dendroids are good 'cose if a very hard troop is next to your ranged attacker, dendroid will bind it to ground. Then the ranged attacker don't need to stay there making just a half damage, it can move and then start shooting. 7th Level troops which are non-upgraded are usually that fast they move just to the middle enemy, and they got the first movement if you situate them at the middle. If they are upgraded, they should put next to the middle guy, so the middle guy can make it and make the first kill to enemy's middle guy.
Middle place is for slow movements very important, that's why an archangel can be anywhere situated.
Move just a little, and wait enemy to move that much you can have the first shot. If your hero has one very fast unit and others much slower, don't send the very fast at the start for killing enemy. In that case all your enemies can in peace just go around your troop and kill it, 'cose your other troops are just on their ways. This one works mainly against a computer opponent, but works just as well with a human opponent. Basically, wait to cast any benefical spells.