You, you unfinished guru, firstly, you wrote that leaving more than 43 gremlins is unrealistic and difficult.
Secondly, with a fairly simple but effective tactic, the battle is won at least 4 times out of 5, with an average of 80-90 gremlins remaining, or even more, so be less of a know-it-all and study the subject =).
Options like "ghosts are slow and attack the ballista, allowing the gremlins to shoot at them" are, of course, not considered a normal tactic, simply because it is not repeatable in general, it depends on the specific AI quirks, and relying on them in online play (I remind you that we are considering a save from an online game, not an offline one) is foolish.
If you were lucky enough to find such an option, well, it happens. My ghosts only attacked the ballista if there was nothing else to attack. Maintaining a normal army with such initial conditions is, of course, impossible.