Sorry if this is obvious, but in case worth mentionning - do you have the back brake mounted and the pedal depressed to secure the disc? If not, this might help in getting purchase. 2 of the other bolts in to hold the disc also worthwhile. In terms of getting purchase on the bolt, in my experience - less than many others - you can normally wedge something in there. An impact driver might work well. I have an old 14.4V Dewalt that works well in low torque situations. A T40 is around 6-7mm I think, and it is worth trying with 5, 6 and 7mm allen bits as well as the torx. Might even be worth trying with a flat head bit in an impact driver if you can wedge it into the bolt head. An alternative is to give it a light tap to get the bit seated with a wooden block supporting the rear of the flange - like Steptoe said, don't swing on it as you don't want to damage the flange. Others can say if they think even a couple of taps is a bad idea. Personally, I would try this before drilling out. And remember a small amount of heat before you turn it.
If you do get to drilling out, you have the option of going from the nearside or offside. I was going to write front or back but thought better of it. If you haven't done this before it is probably worth watching some videos on it to work out how and to see what you will need to get the head off and to remove the threaded length and clean the threads. If possible it would be good to get the threaded length out whole with a ezout or similar so there is no damage to the flange threads. With the head drilled off and the threadlock heated to release, you should be able to remove it either side. You might get lucky and find a drill bit in a pilot hole will rotate the thread out (or even the whole bolt from the rear - clockwise from back is anticlockwise from bolt head end). Worst case you can drill it out from the rear with successive drill bit sizes and clean the threads up. But do be careful doing this as replacing the flange is not something you will want to tackle.