Judging from the fact that the second, G10+ requires 6-star ships, I'm guessing the third tier needs 7 stars. But it's puzzling for new players who can't play any of the three tiers yet and have no ingame way of finding out why that is. Other events of this kind list the requirements somewhere.
0
Replies
The deadly tier does required 7* ships, and unlocks after you beat the third tier (the one which you have referred to as the second tier).
Edit: hope someone else can confirm it unlocks after tier three is completed, I'm pretty sure it does, but mines been unlocked since the first event, and there's nothing on the launch notes from when it was added about it.
Ah yes, there is an easy tier too when I scroll to the left. I only saw the last three.