I found this out as well when obsessively redoing the teleporter maze in Where Only Fools Dare Tread but couldn't get it to work 100% of the time--often he'd move forward just enough the get you within range even if you backed up or start his firing sequence early enough (they don't flinch when hit like headless, ghouls, zombies, ghasts, etc.) The orange crystal works as well but I'd used it up on Oozes earlier.
It's possible even the crystal-less method could work 100% of the time but it's nice to have a use for the blue crystal in the endgame.
: You don't need a crystal at all. The important thing is to back up as soon as
: you arrive, and fire before he has a chance to move. You're just barely in
: his damage radius, just a tiny step away from safety.