The High King Maulgar 25-man raid encounter has an add named Krosh Firehand, which requires a mage to tank using Spellsteal to remove his Spell Shield, and maintaining aggro through modest DPS. The stolen spell shield lasts 30 seconds and reduces the damage of spells by 75%. Krosh is typically the third or fourth of the four adds to be killed prior to High King, so it is your job to hold his attention while the others are downed. Once DPS commences you must continue this cycle until he is killed, which is thankfully pretty quick. You can then join your fellow raiders in the casual Tank and Spank of High King.
Stats that are required are:
About 10,000+ unbuffed health
8,000 to 10,000 mana
6 to 8% hit
500+ or so damage.
This guide is intended to prepare mages with gear from normal and heroic instances. Some items specified are from Karazhan, as it is safe to assume a guild attempting Gruul's Lair would have those items attainable as well. Naturally, if you are progressing beyond Gruul's while still doing Gruul's, there is gear from the higher-level raids that will greatly exceed those listed here. Links to filtered Wowhead searches are provided for each category to aid you there.
Stamina is your focus when selecting gear, but do not neglect intellect or spell damage as you will need it to maintain aggro.
Spell hit is required for a proper set. A 'Nothing to Steal' no-cast does not trigger the GCD (1.5 sec) but a resist does. A double resist on Spellsteal means you could take over 17k damage in 3 seconds, which will likely result in death. Krosh is a boss-level elite, therefore a mage will have a 83% chance to hit with spells without other talents, buffs or gear coming into account. At level 70, it takes 12.6 points of hit rating to increase Spell Hit by 1%, the cap being at 99%. Therefore you need 202 spell hit (12.6x(99-83)=201.6) to reach the cap. Arcane Focus talent makes a big boost in spell hit percentage for spells of that school, which Spellsteal is. With five points in Arcane Focus, you would need only 76 Hit Rating (6%) from gear to successfully Spellsteal 99% of the time. For Alliance, having a Draenei in your group with Inspiring Presence (Priest, Mage, or Shaman) will give you 1% Spell Hit and an Elemental Shaman with Totem of Wrath gives 3% Hit. "/hug Draenei Elemental Shaman" or lucky you for being a Draenei Mage with Arcane Focus.
Fire Resistance is worthless in this fight, as Krosh's spells ignore fire resist. It's a good idea, however, to have Mage Armor on for the increased Mp5 despite the resist not helping.
Here are some things to keep in mind while collecting/refining your set and gathering up your inventory prior to battle:
All of the gear listed below will work, but may not be optimal. Make sure to have at least TWO, level 69-70 "... of Stamina" gear equipped to push yourself over 11k hp. BoE Greens are not listed (there are way too many) but a good one can have up to 84 stamina. There are also BoE blues that can have up to 90 stamina. Other items 'of the Sorcerer' (stam, dmg, and int) and 'of the Whale' (stam and spirit) can be good too. Simply go the Auction House and search for 'of Stamina', etc.
Season 2 Arena set [Merciless Gladiator's Regalia] pieces are now easy to obtain via PvP honor and battleground marks. Resilience doesn't do much for you in this fight but this gear has lots of +Stam, +Spell Power and gem slots. The +Crit will help you with threat generation. Naturally, if you're an Arena fighter, Merciless or Vengeful would be a good way to go. If you're replacing items, consider re-gemming/enchanting the old items for more +stam and +hit. Don't overlook the other arena rewards like [Merciless Gladiator's Endgame] for your off-hand.
Preferred gems are listed as the last part of the gear tables. Shouldn't need to be said but: Don't have empty gem slots! Socket bonuses are often very beneficial but remember that you aren't required to match colors to slots if you don't need the socket bonus.
Enchant everything that can be enchanted or improved with an armor kit, glyph etc. Potential enchants are listed with the items below.