Happens to us too.

We found our way to deal with this problem.

Cross over the purple line at the start of twintanias zone, now the party is on the hill right by the line and looking at twintania.

Have the OT tank move left so that he is nearly facing the last dragon on the left of twintania and once ready the OT runs down and targets the dragons from there, meanwhile have the MT go straight down from the group and pick up twintania simultaneously as the OT moves down.

Hope that works for you as it did for us.