You know that your q and e work well with your ap and your w is good always. Take your opponents to grave with A LOT of mana which you normally don't have but now you would. Using this bonus mana to shield you composed well with your q and ult will make you pretty much invincible. Rabadon is for some extra boost of ip, but it is not really necessary as you will have enough from Seraph's Embrace's passive.

It is so obvious that I don't even really have to explain, right?

Always use your cleanse before activating shield, whilst don't forget to use your Garrison whenever you're diving or they're diving.

You will need a lot of mana to support that Seraph's Embraces, so all runes should grant you mana.