How and why? That sounds like a positioning issue. Ranged should at no point of the fight be anywhere near the boss or the myrmidon. In p3 they can stand on the edge near the Lady Venomtongue - they need to interrupt her while killing Serena (where the melee and the boss are). If you get beckon and spear at the same time, so what? You are far away from the boss and the spear won't hit anything. Nobody should stay anywhere near a player who has either beckon or spear on them, let alone both.
And what about the arcane burst? Are you not dispeling them hidden from the line of sight? You have 20 seconds to do that, or you can just let it expire, again, hidden. Are you getting hit by the spear while you are hidden and so the spear explodes or what?