I've been playing with IG88 arena lead finishing in top 3 for a few weeks now. Was running IG88, IG86, Rey, SF, RG and just switched to an even more fun build with IG88, IG86, Rey, RG, and Vader. Was able to finish today at #1 with it as well. You don't need a tank but it does make it a better defensive team and give you more buffer if RNG didn't go your way even on attacks.
Here is a video for some reference. Just a 5* G8 Vader but I had fun, that's all it mattered. Was still enough to beat meta teams.
BTW, I know there were some questions on DOT damage, from the video and comparing the opponent's HP, I was able to calculate per DOT damage to be 4.99% of opponent's max health. This is a fixed amount damage, it doesn't drop as opponent's health drop. So 4.99% when Rey is at max health and still 4.99% for when she's at 10% so can easily finish a toon off.
http://i.imgur.com/cQ6Ai4S.jpg
https://www.youtube.com/playlist?list=PLtoAQc3PGovx-ny10RMuS4G_i2L_WeSyJ