Honestly, the easiest way is to join a FC or LS with competent, skilled players. I was lucky and the FC of one of my friends in a LS needed a healer, theirs went on vacation for a week. So for a week I was subbing in for them, and we downed all three Ex primals within a week or two if when it came out, and they even took me through Coils 1-5 which I had never done before. I've since joined their FC and any time I need to down an Ex primal we can usually find enough people on in our FC to down them.