I almost died with that big T-rex in the Henne Mines. Although he was just right for my party's average level (Lv25), I got careless and fought him just depending on Larsa's unlimited supply of Hi-Potions.

I was just sitting back enjoying myself until Penelo got killed and Vaan was struggling for HP. I immediately replaced Penelo with Ashe and defeated that stupid dinosaur. From then on I became careful and never became reliant on Larsa's Hi-Potions.

Gambits can be quite a hindrance. I have my gambits turned off except for Foe>Any - Attack. Just that and nothing more. I also have my party leaders (usually Vaan and Ashe) auto-cast Libra for traps. Thanks to Jerseymilk for this tip.