A flat sunk bolt with an Allan head was torn out, the hex key had no grip and the sunk bolt could not be grasped with pliers. It was a fat bolt so drilling out would have been very difficult.
Cut a slot into the screw head.
Place a screwdriver in the slot and tap with a hammer in the counter-clockwise sense to loosen the bolt.
Unscrew the bolt with the driver.
Use the same technique for Philips or crosshead screws.
Use a grinding / cutting disk, not a saw blade. Apply the rotating cutting disk gently, do not force or it will break.
If this is a toughened steel or stainless steel bolt (much tougher), it will be best to use a reinforced cutting disk.
Use a dusk mask to avoid breathing the dusk from the disk !
(No, I don't usually, but I should !)
While this works it is very difficult to not ruin the material around the bolt.
What I do is use a torx bit. size one up that is slightly larger then the hole and then firmly tap it in. atttach your drive handle a turn slowly while pushing down. this will normally break it free.
Also before your get to this point try putting your allen key in the hole and smack it with a hammer. sometimes the force of the hit will cause the metal to "break" enough that the allen screw wont strip.
There are also specifically designed screw extractors made for this. These tools are generally two-sided: one side is used to drill the hole, while the other is inserted to extract. If the process works, the surrounding area is undamaged.
Use another Allen bolt to weld over it and then you can easily remove the Allen bolt with the help of Allen key
And if for any reason if you can't weld then just drill the head of the Allen bolt so it's shears off. And then u can easily remove the rest