The point of the EV is to concentrate your enemies into one spot so you can use your point-blank strike, the strongest part of the charge, to the one area where the slags are, not hit all the charge blasts. When they're compiled into one place, you can do that and keep them right there to do more combos and finish off the whole group without chasing the stragglers...if there's any at all. The shock you dealt with Voltedge (which is extremely likely) will have kept them in the bomb explosion, the EV inflicting shock itself when it explodes so they are guaranteed to be trapped where you placed the bomb, rarely do they fling anywhere when you pair the two together, they just synergy so well. Even someone like myself who has done FSC for years find it to be a great boon in reducing the time you spend in the place, if you memorize the spawns, that's even better for EV because bombs are all about knowing spawns.
I don't know what else to say other than that, I've used both Combuster and Voltedge + EV for extended grinding periods with Max CTR and DMG on everything and I can only say that the latter is better here if one is able to afford to do so. Combuster is only seemingly faster since you get out more charges, but unless the enemies are clumped to get the point-blank strike, the damage you deal is significantly lower vs the combo since you need to run around and take care of stragglers every single time. The extra planning just beats the simple spamming in this place. Sorry. :I
EDIT: Yeah, not much point of the back and forthing as we've derailed the topic heavily. Let's just leave things at that.
Ev has a radius smaller than that of a combuster's hit line, so more often than not I find outlier zombies still around when an ev is used. Also consider when you're using an ev you're not using a combuster. Slags go down very easily with a combuster. Time spent on combustering is smaller than prepping an ev then using a combuster afterwards when solo. I usually can get maximized combuster charges solo because I know where to stand and the pattern they spawn in anyhow, no ev assist is needed for grouping them if you've done vana long enough to know spawn patterns.