For well-geared warlocks (i.e. 450+ resil), as sl/sl the hardest classes to beat are rogues, BM hunters, resto druids, and warriors.
Rogues - it's all about who gets the jump. If they are a good rogue, this is nearly impossible, which is why they are so hard to beat. If you can break them before they CS you, CoEx IMMEDIATELY! this is a 100% kiting battle. Don't trinket ks, it's not worth it. The best thing to do is (if possible) wait for them to blow sprint, then after it's over, trinket crip and put CoEx on them. If they are on you with crip on yourself, switch to CoW. Some of this does depend on his spec.
BM hunters - stay in melee range AS MUCH AS POSSIBLE! Just because he has melee abilities doesn't mean you should avoid them. If you see him lay a trap down, go ahead and try to run around it, but don't do that if it'll get you further from the hunter. He doesn't have any magic buffs, so if he uses frost trap just dispel yourself from it. Save your trinket for wingclip. The trick is to CoW+SL the pet, SL+corr+CoW or CoEx the hunter, then just follow him around and melee him (some damage is better than no damage, right?). If all of your instants are on and you just need to follow the hunter, use all your instants on the pet. If it lasts long enough, you might be able to kill the pet. Do NOT fear with BW up. It might be a good idea to fear them right off the bat to get them to blow BW. After it's over, if you're alive you win.
Resto druid - drain drain drain drain drain!!!! It is 100% about keeping the druid dry. Keep your dots up, drain life when possible, fear while in caster form, drain as much as possible, try to fake him out when he's in bear (i.e. cast fear then stop to get him to feral charge). Save trinket for a 6 second cyclone. The reason they are so hard is because a stack of lifebloom can heal through every dot we have, even with 1400+ damage. And it is sooooo easy to keep a 3s of lifebloom up even with spamming drain mana.
Warriors - ah, the bane of my existence. Good warlock with great gear vs. Good wariror vs. great gear... gg warlock! It is nearly impossible to kill a good warrior even with pre-sacced vw, vw out, and then another sac + vw with FD. You just can't do it. The warrior can be immune to all fears 99% of the time (minus death coil), has MS up so you don't get alot from DC/SL/DL, and you can't drain tank anymore because of pummel. You can't kite because of spamstring, you can't trinket + kite because of intercept, your only hope is that you can deathcoil+fear, he does whatever and intercepts, trinket + run like hell. Even then, a smart warrior will be waiting for the trinket + CoEx and be spamming hamstring after an intercept, even after it's been applied. So, you're pretty much screwed. Have fun!