One of the better Eldar formations in Apocalypse is the Sunstorm Squadron, and this mainly stems from the fact that it costs no extra points to use, you merely need the Fire Prisms to fill the slots. You get a great variety of options you can use for combining shots, and the loss of any single Prism will not have a huge impact on the others.

I'm going to be using a full formation - that is 6 Fire Prisms - in Apocalypse, which gives the Eldar a truly unlimited range weapon to use. It's important to note that otherwise, without a Phantom Titan, you need to slug out fights using 72" as your maximum range. In a large Apocalypse game, this really hampers your ability to fight. The problem is just the larger playing field, and the fact that other weaponry can have a far larger area of effect.