First, toss those broken bolt extractors in the trash. By the time you get those studs drilled and hammer those extractors in, they'll either lock the remnant in place permanently or break off and **** you even harder.
Second, mag drill you say?
Left hand drill bits. Start small and get it centered, size up and drill in increments. 90% of the time they'll either unthread the remnant when they hook up, or they will ream the bolt next to the threads and allow you to tap it loose and peel it out.
Broken bolt extractors are flawed fundamentally and only work if a bolt is broken from being over tightened and having the head ripped off. Because they're hardened, they like to break off and really ruin your day.