Shinobi

The strongest Assassin-type enemy, although technically speaking, it's an slightly improved version of the Stalker. Despite that, this enemy has the same characteristics as it: a high HP count, near total immunity to physical attacks, and to make matters worse, it also has access to the Forced Purge Stored Attack. If it pulls it off, not only your Burst Gauge will get reduced to 0: you also have the risk of the Song Magic of your Reyvateil exploding and dealing a pitiful amount of damage, so make sure to interrupt them whenever they try to use it. As for eliminating them, the same advice given for the Stalker is given here: try to keep them off from your Reyvateil until you can Purge her normally to a high enough level, and they destroy them with a well-charged Song Magic or Flipsphere.