Depends what you're going for.
If it's experience, you want the first encounter on the Hard level to be the Flans. If it isn't, retreat and start again. (Since you can see the encounter beforehand, you won't waste any stamina.) When you get a Flan fight, defeat them, then retreat and start the event over again. Rinse and repeat. Flans give you tons of experience points and are pretty easy to defeat, so you can level up weaker character very easily this way. (Especially if you bring an AoE skill like Dragon or Shiva.)
If you're going for Memories, keep checking the first encounter on the Hard level for the No. 24 encounter. Just like with the Flans, if you don't get it as the first fight, retreat and start again. But unlike the Flans, when you do get No. 24 as the first encounter, you then want to complete the entire Hard event. You'll get the most Memories per staminia this way. (You can do this trick on the Normal level too, but you want No. 128 to be the first enemy for that one.) The reason this works is because the enemy mobs are pre-set once you start the event, and if you get No. 24 on Hard for the first encounter that means the entire run through will net the most Memories. (Same deal with No. 128 on Normal.)