Well lets take entry fight like halfus. A typical whelps + drake release on pull. 8 whelps have 4.1m hp together, drake has 4.1m hp as well, with a typical 2 tank, 3 healer 5 dps setup. If each dps does 12k on whelps, it takes you exactly 1 minute to take them out. Same goes for drake (did not include tank damage here). So you have 4 minutes left to take out halfus his 32m hp. Can assume that halfus has 30m hp left after those 2 mins, from MT damage.

With 2 drakes released/killed you'll do 100% increased damage on halfus. When releasing last dragon that you don't kill, he'll take 150% increased damage. However he'll stun raid 3x in a row with his roar, at least 2 times when he is < 50% health. So lets do the math for 3m40, which means you'll need 125k raid dps / 6 means 21k dps in that phase. Given 150% buff thats very doable. But you're boxing them, and to gear up 5 mages from hc gear, thats taking a while as well. It'll be a challenge for sure.