Palp g11 (L), Vader g10, TFP g9, Tarkin g9, RG g8 - breezed right through all tiers, with the exception of the bonus tier where I had to switch out my previous Zader lead for aforementioned Palp lead, which I can recommended through all tiers if you run an Empire team.
The only reason some people with are having trouble I can come up with are: 1) you have no or otherwise really bad mods on one or multiple chars, 2) Your chars are undergeared (more than 3 are at g8, lower than g8 is unacceptable) and 3) You have not levelled their abilities enough. You might be able to get away with some of these inadequacies at tier I given that your tactics are solid, but it's going to be a rough ride regardless.
Finally, the rewards of this event aren't that amazing. The only valuable drops (that you can be assured of) are Teebo and Chirpa shards and your Teebo should've already been 7* by the time you reach the level to attempt this event. So honestly, even if you can't make it through tier 1, you haven't missed out on much.