How to defeat Vittorio and his ballista?

take the Delab hero, defeat the enemy with their own weapon

That makes sense.

but.. I didn't have any problems with him at all.. I destroyed his army too quickly.. (I generally don't pay much attention to enemy units).
I quickly defeated the army - and won.. that's it.

only the elves are still a difficult opponent.. and the rest fall like leaves in the wind..
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*more than half of the rushing hero's army moves across the entire map..
*it makes sense to destroy such heroes at the very beginning.. it's a huge benefit..