I am not able to think of an expression that conveys both the idea of size and of a single event. The latter seems more important in the context, given that size is implicit in ‘billion’. I do not really understand the poster’s objection to “one fell swoop”, but a less extravagant phrase expressing the same idea is:
at a stroke
Collins Dictionary explains this as follows (my emphasis):
“If something happens at a stroke or in one stroke, it happens suddenly and completely because of one single action.”
One reservation I have about this is the slightly incongruent “placing” that follows in the sample sentence. This would seem a little weak in the context, and I would replace it by a verb conveying slightly more action. “Moving” or even just “putting” might be sufficient — “thrusting” or “propelling”, if one wished to be more forceful.
Another point is the positioning of the phrase itself, which more usually is placed at the end of a sentence. So, assuming that “top band” is unambiguous, the following might be my preferred revision:
The company raised over $1 billion, propelling it into the top band at