Sergio Aguero missed two penalties in the first 20 minutes but rediscovered his scoring touch to bag a hat-trick with David Silva and Nolito also on target in the Romanian capital.
Guardiola again left goalkeeper Joe Hart on the bench, but saw his side dominate from start to finish as they eased to victory to leave Steaua with a seemingly impossible task in Manchester next week.
“We are almost in the Champions League and that was our target at the very beginning,” Guardiola told BT Sport.
“It was a great performance, we played really, really good. The result is amazing but the way we played, I’m so satisfied and it’s a big compliment to these fantastic players.
“All the managers in the world want to create chances and concede few. We are there so now it’s rest and focus on our coming games.”
Aguero’s missed penalties were the only blot on City’s copy book, but team-mate Kevin de Bruyne was not overly concerned.
“We play a good game and it’s a really good advantage for next week,” said the Belgian.
“In the end he scored three goals, so what can you do? If you score three goals nobody will complain about the penalties.”