Yikes, OK, let me see if I remember. When I played on the PC, I was a White Mage. I had just gotten the soul before life took over and I couldn't play any longer, so I focused a lot on the tank. Whenever an add left the tank's and dps's area of death, Tab-targeting usually works with a quick tab->Fluid Aura. Because you're a Lancer though I'm not sure how well tabbing would work. You could make a macro that targets your focus' target:

/assist <focus> which would, I believe, target whatever the tank/whichever is targeting.

The controller makes targeting grouped monsters way easy by just pressing left or right on the digital pad or press L1. I have to assume a similar concept exists on the keyboard in the form of macros OR if you go into your target keybind settings.

I hope this helps and isn't one giant fumble! xD