Yesterday I went around and completed the Spirit Tamers in Pandaria. They were relatively easy. I remembered getting to Northrend and pulling my hair out for days as I struggled to defeat these tamers. Major Payne led to tears. The one who's name I don't know in Dragonblight caused bad words to come out of my mouth.
Here are pets that I've found useful:
Jade Owl - it's magic with flying attacks, lethal against aquatics - it can be purchased in the AH, or get a friend with the jewelcrafting pattern to make it for you.
Clockwork Gnome - although don't use it against a spider with brittle web - each shot of your turret damages you
Flayer Youngling - Take the time to go farm a rare one. Humanoid with beast attacks will destroy critters (and most anything else as well) if you don't want a flayer youngling, then get the kun-lai runt or the stunted yeti
Yellow-Bellied Bullfrog - any frog really - get a couple. They have the flies and the tongue lash that are good attacks vs undead plus they heal themselves nicely.
Celestial Dragon is a great counter to flying pets - takes some pet collecting to get though, so if you're not there the Nether Faerie Dragon is a good option.
Snarly - this is a pet you get from doing fishing dailies outside Shattrath. This is my first pet to lvl 25 and I still use him all the time.
Water Waveling - everyone has their own favorite elemental, this is mine.
Rapana Whelk - strong vs elementals. i know you're talking about northrend tamers, so I don't know if you're lvl 90 and can get to the Dread Waste in Pandaria, but if you can then this snail is great. If not, then darkshore and ashenvale also have some snails - you'll just have to spend more time leveling them.
I don't use beast, because i have a humanoid with beast attacks.
Ok, I didn't really mean to get that detailed, but it just sort of spilled out. One other thing I'd like to mention. . .GET RARES!
I was unable to defeat Major Payne until I got a rare mechanical pet. My uncommon mechanical squirrel was no help.
Spend the time to farm for them. Don't be satisfied with an uncommon fluxfire feline, keep farming until you get the rare. don't use a stone on a FFF - first of all, I personally have never seen an upgrade mechanical stone yet, and I've seen a lot of stones and second, they're farmable, why waste a stone on something you can get.