To make the attack bearable you need Gardevoir... oh wait...
The number of silly, inviable but undeniably hilarious deck ideas that this card spawns is enough for at least 5/10 for me.
If you really think it's playable though, I wish you well.
Gardevoir, eh? That's a good one. xDDD I do wish she was that good, though.
I can't say i've ever seen any real use for Slaking. Which includes it's presence in the Anime and VG, as well as the TCG. It just never seemed that good or impressive.
Card-wise, though, it's current incarnation is pretty painful. It's attack is far from the best, but it isn't all that bad. It can be powered up pretty quickly with the various methods of energy acceleration granted to us (i.e. Emboar, DCE, Eelektrik, Terrakion EX, and pretty soon Blastoise.). 100 is enough damage to 2HKO any non-eviolited EX and a guaranteed discard of one of their energy isn't bad at all. It's high retreat is annoying, but not unworkable.
But, what really kills it is it's ability. So many of the more popular cards and most commonly seen pokemon now are basics; EX or otherwise. Being unable to do anything to them is not a good thing. It gets even worse seeing as a lot of the popular stage 2 pokemon can 2HKO Slaking, or even one-shot him on occasion, and tend to be harder hitting, have a faster set-up, or can all but ignore the energy discard (i.e. Empoleon, Garchomp).
Garbador can shut off Slaking's ability, but having to run him takes up far too much room in a deck which will already be full of things to help get out, and get powered, your Stage 2 attacker which will make the list far too clunky to be reliable. Garbador isn't even a decent attacker in it's own right, so if it gets trapped active and you can't find your switch it'll give your opponent even more time to safely set up on you.
It may make for an interesting fun/league deck, but in the competitive sense it has far too much going against it to really prove useful. It doesn't hit for quite enough damage to warrant its 4 energy attack cost, has an ability that no card should have in this current format, and doesn't even hit any of the meta cards for weakness.
I give it a 3/10