On your question about marking, there are usually groups of 3 mobs where 1 is a "main" mob and the other 2 are kinda minions as such. What I mean by this is that 1 mob will use an AOE move of some sort and have red markers that are usually conal that the DPS/healer need to stay out of.

You'll learn which mob is which when you run the dungeons for the first time and can use that knowledge later on when you run the dungeon again. I usually mark these main mobs first as the others don't need to be downed as fast, in the case of there being 2 main mobs which does happen, just mark them as 1-2. It's perfectly fine to not know the most effective kill order on your first run and if people kick up a fuss, make them mark.

I play on the PS3 so use the cross bar and keep L+directional buttons 1, 2 and 3 for marking 1, 2 and 3. I keep cure on the 4th one so I can top myself up if theres a big move coming up and helps hate a little. Never had a problem this way.