It shouldn't be at all difficult with an M8 (which I think is what a crank arm bolt is).
Given it's an HTii arm, aren't these the pinch bolts tangential to the BB axle and more like M5?
My mistake, and you are quite correct.
I've managed to extract a 5mm (or it could've been a 4mm) sheared bolt from a saddle clamp using the ACP method.
The thing with using a stud extractor is that if you manage to snap that, you are giving yourself a headache in a completely different league.
Using the ACP method you are unlikely to create much collateral damage.
The bolt head sits in a counterbored hole and the other end of the bolt is recessed so it will be difficult to get at it with a centrepunch. If it is the outboard bolt that has failed then you should be able to remove the retaining pin thingy and you then have access to a couple of millimetres of the threaded portion of the bolt through the split that you pinch up with the bolts. I can get enough purchase on this to rotate the bolt using a pair of knives* although it would need to be pretty free running for this to work on yours.
*other bodges are available
You only need to target the ACP marginally off-centre of the bolt for this technique to work.
Obvs. targetting the ACP as close to the circumference is beneficial/efficient, but by no means essential.
It is deffo the low-risk variant.
ETA: I've recently acquired a 'Damaged Screw & Bolt Remover' set.
https://www.higomore.co.uk/products/damaged-screw-extractor-set-of-4?variant=158271020&gclid=Cj0KCQjwmIuDBhDXARIsAFITC_4tPeH5CAaSo6KO0oOjAPmgm-CGcVwqSmkRJMqhvpsdONxULe9b5koaAi9KEALw_wcBI've yet to use it, but it strikes me as being a bit easier to deploy than drying to drill through the centre of a bolt to then counter-drill using an extractor.