Sausaletus Rex

Blackout locks your own skills, too. I wouldn't try and combine it with Wastrel's because you'll need to use Wastrel's before Blackout and that give the opponent a bit of time to use a skill and, in any event, locking your own skills is not a good thing (unless you're something like a Warrior/Mesmer or Ranger/Mesmer. Then it's not actually so bad because you can rely on your normal attack, especially if it's been buffed up, for the duration of Blackout) and not really worth the little damage you'll get from Wastel's.

The most common way people use it is with energy denial - when they have no energy they can't use many skills. Something like a Mind Wrack/Wastrel's combination seems to be the most popular way of going about it. Mind Wrack damages someone if they energy hits zero. Wastrel's damages them if they don't use a skill. Using a skill will likely cost energy and might drop someone to zero, triggering Mind Wrack, but if they save their energy they'll trigger Wastrel's. Of course, somethign like that only works if you've already figured out a way of getting someone close to zero energy in the first place and only if your opponent is using skills that all cost energy - no signets or adrenal skills. So, you'll need something like Debilitating shot or Power Leak or Energy Drain in the first place.

Personally, a Wastrel's+Mind Wrack combonation is forgettable because if you've already got a way of reliably dropping someone to zero energy consistantly then you've already won the battle. Wastrel's+Mind Wrack do alright damage, and they ignore armor, but they don't do insane damage, so I don't think it's worth the slots. I'd rather make the job of denying energy easier than figure out what to do once I've denied energy - an opponent with no energy is beat, they're out of the battle for a few seconds at least, who cares what happens to them then, let the damage dealers figure out how to put them down, I'll figure out how to get them there and keep them there. Wastrel's, for that matter, is a bit of a wasted slot, too. People will always be using skills so it's easy to cancel. You have to work really hard to make it work and the payoff isn't that great. There's other ways of things than energy denial, too, of course. You could do knockdown or interruption to keep someone from using skills - if I recall correctly Wastrel's is canceled not when someone first clicks on a skill but when that skill is actually activated, it needs to go into effect to cancel the impending damage so you dn't want to cast, say, Restore Life to get rid of Wastrel's. Something like Gale+Wastrel's gives your opponent very little time in which to activate that skill but, again, personally, when you have someone knocked down there are a lot better things you can do to them than 8~53 armor ignoring damage.

Not many from the Mesmer side of things, maybe, but as a Mesmer you're not really concentrating on dealing damage, anyway - it's nice and all but there are more important and critical things you provide to your party. I'd leave Wastrel's out and save the space for something a bit better.

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In my day, we didn't have virtual reality. If a one-eyed razorback barbarian warrior was chasing you with an ax, you just had to hope you could outrun him.

Insomnia

Yeah i was kinda thinking about lots of atribute points in inspiration for a good energy drain and another drain skill, followed by mind wrack and wastral's worry. WOuld work good vs profession with little energy, but you're totally right that someone with no energy in most cases already is screwed. I'll have to look for some dmg spells in my secondary profession, focussing little to much on the mesmer thingy...

Sausaletus Rex

Going Inspiration for energy to fuel whatever you're doing in your secondary is a tested and time-honored strategy. Depending on what you're playing you can do a lot with it. If you're looking for damage, you're out of luck in Inspiration, though. it's a defensive/disruptive line, you won't be able to touch a drop of your opponent's hit points. If you're really looking to Mesmer for damage, I'd turn to Illusion. You can do things like Migraine or Phantom Pain. Hexes that will causes DOT - Mesmers are in no small part all about making their opponent useless with hexes, rather than trying to directly damage someone you can work to make them much softer for the people in your group who are actually doing damage, denying energy so they don't have any way of casting defensive or healing skills, stealing away their enchantments, and then piling on hexes. You'll slow them, DOT them, lengthen their casting times, harm them for attacking, and so on,, so that the Rangers or Warriors or Elementalists can finish them off. Even something as simple as spamming away Conjure Phantasm will add a bit of damage to your character and force the opponent's to scramble with hex removal in addition to standard healing.

Or, as you said, turn to your secondary. Unless you're Mes/Nec or Mes/Mon you should have some pretty impressive options to rely on.

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In my day, we didn't have virtual reality. If a one-eyed razorback barbarian warrior was chasing you with an ax, you just had to hope you could outrun him.