Now that most of the battles have gone through, and the round is over, He're what I found with this event. It was a great way to kill some of the 'less useful' cards in the grid, it came at the right time, and it gave picking incentives over flat out grabbing the 'best' cards + relic slots. Very well made event.
The only sad thing I found was the lack of motivation to pick a grid entirely of ninja slots. I had one set up, and it came out to have some okay decks, but the +3 relics for all ninjas was underwhelming. The other events had picking a sub-optimal set of cards from the grid give a good advantage in relics (round 5 for best example). I could use my all ninja setup, and gain 12 relics (6 for 12 ninjas, 3 for ninja targets, 3 from all ninjas) or go with a better deck setup and gain the same amount. (what I choose to do in the end)
Perhaps, you could make the ninja picks amount to scale with relics won next time? Something like first four ninja picks are 1 relic per 2, next 4 are one relic each, last 4 are 2 relics each. That would give incentive to add more ninjas vs only relic slots. Most of the ninja slots were 'sub-optimal' and such a scaling would could make picking more interesting then the 'pick 4 ninjas, 3 to target' setups almost everyone used in the end.