Well, obviously NO mix is going to be invincible, that's the whole point of balancing the game. You should base your units on what the enemy has, and take signs of what's to come with proper scouting. Example: If you see more than 3 Statues, you know Destroyers are coming soon. If you see Chimaera Roost, you know Chimaera are coming. In those situations, melee should be minimal, most of the time non-existant.
Orc vs UD: Vs. Mass Fiends - Chieftan/Tauren/WDoctor/Berserker works well when you spam wards. Also, remember to position your Tauren Chieftan to get the full effect of War Stomp.
Orc vs HU: Vs. Rifle/Caster - Far Seer/Chieftan(max aura first)/Shaman/Tauren/Grunts/Demolisher
Orc vs NE: Vs. Hunt massers - Far Seer/Naga Sea Witch(Shadow Hunter on non tavern)/Grunts/Demolishers.
Orc vs Orc: Vs. Wyvern - Far Seer/Naga Sea Witch(SH)/Grunt/Raider/Shaman/Berserker
Vs. Grunt/Tauren/Caster - Far Seer/Naga Sea Witch(SH)/Grunt/Tauren/KODO=KEYTOWIN/Shaman
Vs. Gruntapult - Far Seer/Naga Sea Witch(SH)/SPEEDSCROLL/Grunt/Kodo/Shaman/Raider
It's a general list of common matches. Do not follow word for word, and do not hold me responsible for losing despite using one of these mixes. I'm giving you a suggestion only, from here on out it's your business to micro, not mine.