How many is "A bunch"?
There's not a lot you can do to screw over a player for being vague. Because the rules-provided answer is basically "Force them to be less vague."

Really what players are doing when they say they make "a bunch" of tokens is telling their opponent "Look, I have my combo. Gonna scoop now?" If you do anything other than scoop, of course they're gonna play more tightly — since now they're actually playing it out.

Get in the habit of always asking for clarification. Then you're not giving away any information about your own plan.

About the most "underhanded" things you can legitimately do are:


Ask really casually and hope they say something a little more definite.


  "So, that's what, a million damage heading my way?"
  "Yup."
  "After blockers, I'll Rakdos Charm."


I think most competitive players would call a judge on you and you'd end up in an annoying argument about who exactly said what when. You might also develop a reputation for pettiness of poor sportsmanship.
Ask someone to slow it down and show you the combo steps. As they do so, they'll probably announce a specific number as part of a shortcut. (This works better with Rakdos Charm than Batwing Brume, because you can do it while they execute the combo instead of having to wait for them to declare attackers.) This is a trick that may actually get you some wins against the more complex combo decks, when they just straight-up botch their combo.