How about some Mesmer skills like -
Psychic Distrations which has a 2 second recharge, faster then mystic regen's
Power Leech, every time they cast a spell, which is all these E/D & A/Ds do, they lose 5 energy, no energy, no more enchants
Migraine, makes all spells have a longer casting time, easy to interrupt then
Panic, -2 energy regen, eventually, they way they spam spells, they will not have energy
Enchanter's Conundrum, enchantments take longer to cast, which makes them easier to interrupt, and if they have no enchants when the hex is done, they take dmg
Power Flux interrupts a spell and they get -2 energy regen
Air of Disenchantment kinda like Enchanter's Conundrum, but when it ends they lose an enchantment
and these are just elites, there are plenty of non elite skills that can mess up these Mystic Regen E/Ds & A/Ds, and there have already been plenty of people telling you Ranger, necro, things to bring. heck, Sins have some Enchantment remover skills too. The thing is, you already know that about 90% fo the time you are gonna face this build, bring the counters for it.
S

Seriously, has anybody ever been killed by an E/D? Just pressure em till he falls, or nerf the skill that makes him take 0 damage from most attacks, not the generic regen that will be replaced by another generic regen
Houstin, we have a problem, and it isn't Mystic Regen.