While you don't completely trust the elves arcane ways despite being allies with them throughout the war, you admit the plan as clever and decide it is the wisest choice of action. Soon, three functioning ballistae are found, but no spell-based traps with them. These will, however, prove very useful to you regardless and you are grateful for the help. This may indeed raise your opinion of the elves as a whole in future days.
If you live to see them, that is.
Now the opposing forces are much closer and soon find themselves being fired upon by your forces and several archers (you were granted few). Of course, their numbers are too great to stop completely, but many of their men are taken before they reach mid-ruins and begin to return light fire. Now, the only plausible option left is...