The best small strap wrench I have found is a Lowell S6 but...
The best way I have found to open first run ARC LS heads is to drill a hole the diameter of the LS head in a block of wood and then cut it in half giving you a couple of vise pads with half of a hole in them. It is important that the hole is a good fit so you don't bend the head oval. Repeat the process with a thinner block of wood to make pads that fit the bezel. Wrap a bit of rubber tape around the head and clamp it firmly in a vise using your wood blocks and leaving the bezel exposed. Clamp the smaller blocks around the bezel after applying rubber tape ( a wooden "hand screw" clamp centered over the light works well ). Unscrew... The head or bezel probably will slip in the pads, tighten clamp/vise as needed, or apply heat and re-try. It is much easier to break the locktite bond if the head is warm, but doing all this fiddly clamping and taping before the light cools off is not easy, so I sometimes heated the bezel with a heat gun a bit while the head was already clamped, this can have bad effects on the optic if you are careless...
WARNING! ARC LS 1st generation heads were assembled while the potting on the circuit boards was wet. It is about 50/50 in my experience that the board will come out attached to the bottom of the bezel. Prying it off will result in components being pulled off the board, usually the inductors or capacitors. Not an issue if you were planning on replacing the electronics too...