Level up to 4 early if you want to start a win streak. Level up to 5 to try and continue the streak.
If you're weak in the early game, you can opt to save gold and lose streak instead.
At Round 2, level up to 6 regardless of your start. Put together the strongest team you can.
At Round 1, level up to 7. Roll down to stabilize your comp if you are weak.
Once you are back to 50 gold, begin slow rolling for Shyvana and Nunu 3. Once you find them, level up to 8 to complete your team comp.
It's unlikely that you will hit level 8 a lot of games as you slow roll for Shyvana and Nunu, so it might be difficult to find Sett consistently. You may have to settle with only Shyvana 3 so you can find Sett and have your 8 Brawler synergy. In that case, itemize Sett over Nunu.