With several exchanges, you can pump up an evocation pretty high.
For starters, assume that with the extra time to gather the power you are gathering the maximum you can without inflicting consequences. So conviction +4, typically. Add in some navel gazing maneuvers for the control roll, and the target is unawares, so gets a mediocre defense, you are talking 18 shifts easily. If it is a mook, it doesn't automatically get to take beyond moderate consequences to avoid being taken out.
Hmmm. Maybe have a rule that mooks don't generally take more than one consequence at a time. So instead of it soaking mild, moderate, and severe all at once, the most it could take in one shot would be a severe.