LogFAQs > #966717213

The EX1 dungeon (Tideborne Tower) is when the developers got tired of people using healers to cheese boss mechanics rather than using multiple teams. So they started putting in hard enrages (aka party wipes), in order to force people to build more characters. EX1's boss enrages around 10s left on the clock IIRC, while EX2's enrages when there are 20s left.

You shouldn't worry too much about immediately clearing a dungeon after you unlock it, as generally the dungeon boss will be much higher level than you at that point. Most players will need to build up for a time before they can clear their newest dungeon boss. As long as you can clear all of the trash packs in the dungeon before the boss, you will be getting more coins and mana than fully clearing the previous dungeon.