well, i use invisiblity all the time. On warcraft 3 get a army and a hero with mass teleport and a cloaked unit and send it into the enemies base, use teleport, then BOOM your whole army is in there base.
On warcraft 2 i use invisibilty a LOT and it has gotten me over 800 victories on benet when it WAS popular to play-
Get about 6 Mages and have them al caste invisiblity on eachother and be sure to have blizzard reayd and enough mp. caste invisiblity on eachother and walk into the enemy base. send 2 wizards each to a vital enemy base location, like mining area, barracks, farms, etc.. and caste either blizzard or fireball. i recomend one use blizzard while the other casts fireball and let it also caste firesheild (or is it sword?) on either closely packed enemy units or on your mages if they get into trouble.
or if you want send all your mages to one area and have em all fireball it to death damaging other buildings and peasents and maybe even troops too, or just reate a massive blizzard.
also while ur mages are doing that have a army of AT LEAST 10 footman and 10 archers and have em just rush in. this might not kill ur foe but itll surely damage them like hell.