Get a competing job offer. Whatever they offer you, ask for a comparable raise of your current workplace. Be very explicit about it being non-negotiable. Whether to mention the competing job offer is a matter of taste; it helps prove you mean business, but it can damage trust. Either way: if they say no you leave.

This only works if, and because, you are ready to walk away. It's the easiest form of negotiating. But it does require you to actually be able to walk away!