So the items are for magic power mainly, with a little extra health and mana, and a bit of magic penetration. I don't think it shows it but the Rabadon's Deathcap adds a 30% magic power which would top off the magic power at 500. With Time bomb maxed out it will be about 800 damage.

Basically you time bomb your victim, maybe slow them down first, Rewind, time bomb again this one will automatically set the first off. With as much cooldown reduction you might be able to get rewind back in 3 or more seconds to repeat doing 2 time bombs at a time. The main goal is to pump out as many time bombs on your target as possible.

I am kinda new so I basically just chose runes to increase magic penetration and cooldown.

Again, like the runes, I was focusing on magic penetration and cooldown time.

Basically it gets the time bomb up to almost 1000 damage and gives you decent health.