I suspect this wording won't work.

I think it works enough for an Un-set, but I think it'd cause the Rules Manager to start gibbering in black-bordered land.

I just want something that triggers if you hit it with untargetted removal, really.

You might be able to do something like "Spells and abilities treat ~ as if it were phased out. A player may pay 2 to ignore this effect for the resolution of one spell or ability."

Which works for that - but I want this to test the wording towards a creature that counters mass removal but can be singly removed.

How about something along the lines of this: Counter any untargetted spell or effect that would affect CARDNAME unless the spell or effects controller pays ???

Or this: Obstinate Lurker cannot be affected by spells or abilities that do not target it. Spells or abilities that target Obstinate Lurker cost more to cast and/or activate.

How about just "~ is unaffected by all spells and abilities unless their controller pays 2"? The problem is whether what counts as "affecting" it is well defined in players intuition and in the rules, and if those agree -- but do they? Would everyone agree that "gains flying" or "must block" or "can't be blocked" is an effect, but "number of creatures in play" isn't? The tricky thing is it harks back to the problems with the earliest versions of "protect": if effects refer to it without DOING anything to it, like "creatures you control can't be the target of spells and abilities" -- would that effect apply to this? I think it clearly wouldn't unless you pay. I'm not sure, and it's probably still "un" anyway, but I think it I think it may be clear enough.