Take 'Em All On!

This achievement appears to be closely related to Family Reunion, so I'll use the same list I posted there. It counts battles against wild pets, and appears to count PvP battles. It does NOT count battles against Master Tamers.

Update for pets added with 5.1.

Aquatic
Various low-level frogs & crabs all over the place.

Beast & Critter
Every starting zone is crawling with them.

Dragonkin
Spawn of Onyxia @ level 12 in Dustwallow Marsh near Onyxia's Lair.
Infinite Whelpling in Tanaris @ lvl 14.
Used to be Flying type, but now a Dragon: Nether Faerie Dragon in Feralas @ lvl 12.
Nexus Whelpling in Coldarra (Borean Tundra) @ lvl 22.
You can also get Wild Crimson hatchling, Wild Golden Hatchling or Wild Jade Hatchling but they require you to be level 90 and exalted with the Order of the Cloud Serpent to even have a chance to catch the level 22-24 pets.

Elemental
Ruby Sapling in Eversong Woods.
Tiny Bog Beast, Wetlands
There are a lot of Ruby Saplings in Eversong. Tiny Bog Beast seem to be fairly rare in Wetlands. Alliance characters might be better off making the trip to Eversong.
If you can't find either of those, there are elementals all over the place in Deepholm at levels in the low 20s.

Flying
Flingin' Flangin' Moths. Can't get rid of them.

Humanoid
Harpy Youngling in Northern Barrens @ lvl 4.
Stunted Yeti in Feralas @ lvl 12.
Flayer Youngling in Hellfire Terrokar Forest @ level 18
Qiraji Guardling in Silithus @ level 16 (Only during Summer)
Sporeling Sprout in Zangarmarsh @ level 18

Magic
Lofty Libram in Hillsbrad Foothils @ lvl 7.
Jade Oozling. There are a lot of these inside Skulk Rock in The Hinterlands.
Arcane Eye. Deadwind Pass @ lvl 18.

Mechanical
Rabid Nut Varmint 5000. Lots of them in Azshara, or Stonetalon Mts. Even if you don't see a wild spawn, they show up frequently as the 2nd pet in Stonetalon.
Tiny Harvester in Westfall. These seem to be fairly rare and only spawn in a small area so are likely to be heavily camped at first.
Fluxfire Feline in Dun Morogh @ lvl 2.
Anodized Robo Cub near Everlook in Winterspring @ level 18.
Cogblade Raptor in Blade's Edge @ lvl 20.

Undead
Infected Fawn or Infected Squirrel in Bloodmyst Isle, Silverpine Forest.
Lost of Lordaeron, Tirisfal Glade.

I think the "win a solo battle" means that you engage a pet of every family to a battle that only one pet is your opponent, not 2nd one or 3rd one

Edit: I just got credit for mechanical! I fought a [npc=62119] in Azshara against a 2 pet battle but the only difference it had is that it was the first pet out! So to get credit for this achievement you must fight one of each pet family when it is the first out!

Anyone think this should be "Take 'Em All On"? The apostrophe before the "Em", and not after?