So for the individual achievements on the Mystic Titans are in percentages but the alliance achievement is set numbers. In my opinion the lowest bar is set way too high. Too lower the bar but also keep it high I think the tier structure should be a hybrid approach. So keep what you have now, 1, 2-10, 11 - 100, but then go to top 1%, 5%, 10%, 25%, 50%, 75%, then everyone else.
Honestly I think this should be the approach for all events, with free avatars (backgrounds/pins) for the top number brackets.