Hear me out
I think if Cecil actually locked in he (or rather him and the gda) could be viltrumite levels strong.
So Cecil could dodge Omni mans attacks in season 1 using the teleporter, all viltrumites seem to have about the same speed so you could reason he could survive an encounter with conquest or maybe even thragg, and do actual damage with the Infinity Ray (he'd probably only shoot one shot to minimise damage) if he could stun a viltrumite with the sound tech which is like never used again.
The main problems here is with the teleporter reaction time and the colteral damage from the Infinity Ray, as well as for the sound stun to work they would need to be on a planet or at least somewhere with an atmosphere for the sound to travel through (unless they attached bone conducting speakers or something), the reaction time for the teleporter would be a big issue as in the Omni man fight? Intervention? Cecil or rather the Al controlling the teleporter
can see Omni mans movements ahead of time.
This probably wouldn't work with thragg aura framing so some kind of trap would probably be needed so Cecil probably rage baiting thragg from a distance.
As for how Cecil would acquire the Infinity Ray? Ask nicely? Maybe get atom eve to copy it