Once you realize that the magic penetration from Sorc boots stacks, the next logical step is to abuse it! You should focus on getting as much magic penetration as possible. What that means is you should invest in overly-expensive quintessences instead of buying other champions you can enjoy. Well, you probably got to your current ELO by abusing Swain. If you don't want to buy 6 shoes, you should at least get 2 for both of your feet. With runes + masteries + items you can get a total of 110 (or so) flat magic penetration, which is a lot. like wow. You'll essentially be dealing true damage to the enemy team.

Well, I can tell you that you need a smite. The other spell is up to you, but you should always keep a smite on you because you probably suck at last hitting. You could pick up a teleport, but since you have smite you can always push up lane before leaving. Another option is ignite - yay more true damage.

Your two viable options in the cunning tree are Windspeaker's blessing and Stormraider's Surge. If you want to live longer, you get Windspeaker's blessing; If you're confident in your true damage assassinations - Stormraider's Surge. Don't even think about Thunderlord's Decree, that's some p*ssy sh*t.