...dude, did you miss the 4hp every 0.5s part? when she's not fighting, so if she gets 10s break between fighting 2 mobs, she gets to recover 80hp, or 120hp after cc, which isn't much to rely on, so fighting back-to-back obviously isn't best for her.
edit: might as well just throw some videos in, taken from JuneJuly at Aigis Wikia:
Rikka as a frontliner http://www.nicovideo.jp/watch/sm26637494 or http://www.nicovideo.jp/watch/sm27558871
and to show that her regen is not as fast as poison, http://www.nicovideo.jp/watch/sm27787501