This comp revolves around Veigar's powerful ability to one shot units that are lower star level than him. As such, Veigar is a priority to 3 star. While 6 mages might be better, it requires you to highroll more and shadow will provide a much needed boost to your midgame.
This comp is somewhat unreliable as it relies on a 3 star Veigar, but with Neeko's Help and a strong mage start, it can be quite feasible. Mountain Taliyah is also a big help to run this build as an early mountain buff can help the early to mid game while you save up eco to find 9 Veigar.
The early game can be quite flexible, but early woodland units provide the best transition into the midgame. Simply try to be as strong as possible while spending the least amount of gold.
In the midgame, you are only 1 level off your core build, but being able to put in early 3 shadow will help your damage more than adding in 4 ocean too early. If you can't find the shadow units, however, you can add in Thresh and Nautilus at 6 instead.
Since this comp revolves around Veigar, you will likely not level up to 8 and instead continue rolling at 7 to find Veigar. In the meantime, you can try finding other upgrades like Thresh or Kindred if you feel like you have enough gold to spare.
You can put Seraph's Embrace on Kindred first to win streak and when you find Veigar 3, you can replace Kindred with any shadow unit.
If you are fortunate enough to hit level 8, you can add in mystic with Master Yi and any mystic unit.