Me/W Distortion/Bonetti's

My current Me/W build is the following, with an emphasis on the Distortion/Bonetti's combo.

Illusion 4 [minimum to make Distortion cost 2/hit]
Axe Mastery 8
Tactics 8

Cyclone Axe [+8] [Flurry for PvP]
Executioner's Strike [+26]
Penetrating Blow [+13]
Bonetti's Defense [8sec]
Distortion [2e/hit]
Healing Signet [100h]
Sprint [8sec] [Imagined Burden for PvP]
Rez Signet
[as you can see, an IW build in the making]

I find that I can easily hold up in large melee mobs [especially Ettins] by doing the following:
Aggro, Distortion, and Cyclone Axe.
Spam Distortion, use Cyclone Axe only if you're adrenaline skills are low.
Spam Healing Sig if you're taking lots of damage, making your only source of damage Cyclone Axe.
Hit Bonetti's as soon as your energy hits 0.

However, as good as it is against mobs that rely on attacks for damage, I find myself getting raped by mobs with spells, for Distortion can't stop it and Healing Signet forces you to take double-damage. Does anyone have any clues as to what I can do to reduce the threat of spells?

Well you're a Mesmer. Naturally you have tons of options for reducing the threat of spells, but most of them require extra points into Domination to be particularly effective. Even without points into Dom, there are plenty of skills in that tree which can really hurt enemy casters and reduce their ability to harm you. For instance Power Leak is a standard spell interrupt which drains the enemy of 10 energy even with zero points in Domination.

Warriors also have plenty of anti-caster skills - Disrupting Chop, Hammer knockdown abilities, and Savage Slash just to name a few.

With your current attribute layout you're just naturally not going to have a ton of really strong options for countering casters (which seems natural, given how effective the build probably is against attackers.) Pumping points into Domination and using some of the abundant anti-caster skills in that tree is going to be the simplest way to solve your problem, but naturally you're going to sacrifice some/all of your immunity to physical attacks in doing so.

The obvious thing to do is take points in Domination or even Inspiration to give you some anti-caster options, but this will just split your resources and make your build less focused, and therefore probably less effective.

Sticking within Illusion, there are a couple of skills that although not anti-caster, could help defend against them. I would recommend Illusion of Weakness, for a vital boost in health when most needed, and Illusion of Haste to help close with casters to deal with them straight away.