Maybe then not something like Sheremetev's post with "+"s but checkboxes instead and an "Apply Potions and Fight" button next to "Fight" that would apply them and go straight to battle instead of just refreshing the page.
In any case, I give my full support on this suggestion.
Less page reloads = success.
P.S.: I'm taking a little too far maybe, but the game could show us a box with each merc's HP/MP when hovering (or clicking) the mouse button over (or at) each merc's name.
Edit: Instead of an automated system, there could be some small settings menu that would allow players set what regenerate item each hero/merc would use. In that case, it would be more useful to have a numeric box* there instead of a checkbox so the player could multi use potions as well.
*I don't know if that's the right way to call it, I mean a box where the player can type a number.