If your team has 2 targets of one element, then the enemy (with an element superiority) attacks the one with less HP. If he attacked another target, it probably rests on the mechanics of the AI of a certain character. For example, S3 A. Cidd attacks a target with the highest CR, or S3 Spez chooses a target with a stun as a priority. Etc.

 So it would be nice to be specific.

Yeah the Ai has 2 (or more) roll tables. The most common one is referred to as “Play to Win” and if it chooses this table you will lose unless your team was already going to win regardless of any counter/dual attack and skill/artifact procs. Under “Play to Win” the AI will ignore the normal “random” AI behavior and follow a different set of parameters.

For example, R.Violet S2 targeting is adjusted to prioritized stealthed units when the “Play to Win” table is selected.

Also if a it calculates that the teams full damage potential is less than the needed damage to win before the player reaches the damage potential needed to win, it will add in dual attacks to ensure the win.

There is an official document on the E7 Datamine site.

E7 has a gambling mechanic and yes like all gamble it's RIGGED