The best way to optimise this event, is save a bunch of low level dragons specifically for these tasks. if you pick dragons you were already going to level up with food, then you cant go wrong!
or if food is not an issue - you could power up some low level Terra dragons you hatched in the previous missions.
the lower the level of dragon the less food it costs - but it takes more luck to get items. I find powering up level 1 dragons to level 6 takes little food, but can give 5-6 items.
One thing I do outside of these types of events is leave all my newly acquired dragons at level 1 - and only power up dragons when the tasks for feeding appear in events!
This is a great way to complete the task, because these dragons needed powering up anyway