Around the 30 second mark is when players normally gain full concentration, Timewarp is used and every cooldown every player has is used. I know myself as an arcane mage I can easily jet up to around the 200 to 250% threat marker during this time. Which mage invis, is awesome for.
As big pointed out, you want to pre-pot strength, reck, shield block, inner rage (for more heroic strikes), and keep SS on cooldown. Your goal is to build a massive threat lead.
If this doesn't work:
Use your mages. Tell them to delay invisibility and after mirror images fades taunt the boss and jump way up on the threat table. This will take some trust, because seeing a giant flame cat run at you is pretty scary, regardless if you knew it would happen or not.
[Today 06:48 PM] Ion:swimming in a natural body of water ISN'T acceptable...it's momentarily tolerable