If you want Aveline to get aggro from the melee attacking Bethany in your scenario, then you can try
1.Bethany: Health >75% -> Move to Tactic 3
2.Bethany: Being attacked by melee -> Taunt
3.Enemy: Using melee attack -> Use condition for next tactic
4.Enemy: Attacking Bethany -> Attack/Bash/Whatever depending on how you've build her.
What this will do, is that when a melee attacker hits Bethany, Aveline will target him and attack him. If Aveline can't get aggro with her autoattacks and the enemy continues attacking Bethany, when she reaches 75% health, Aveline will taunt.
This works cause the act of Aveline attacking a melee enemy, makes her move near Bethany, so when Bethany's health drops, Aveline will already be in position for her Taunt range to reach all the melee attackers. The only situation where this tactic fails is when Bethany takes too much damage from ranged attacks taht triggers the first tactic forcing Aveline to taunt on the spot, so you should be careful with this to not run ahead with the mage before your frontliners get aggro from the archers or without some form of defensive cooldown.
Like for example, add to your support mage a line
Bethany: Being attacked by melee/ranged -> Cast Barrier
Or for your rogue,
Bethany:Any -> Chameleon's Breath