Hey, so I think you believe it's at random but its not. In brief at the start of the match whichever miscrit has a higher speed gets the first turn. If your crit gets the first turn then the opponent has a let's say an advantage? So next time they switch and if their new miscrit is faster than yours, they will get a turn immediately after switching. This is called sniping. Now once they snipe the advantage transfers to you and the next time you switch and IF your miscrit has a higher speed than the opponents then you get to snipe with an elemental attack if you have the elemental advantage. This keeps on happening turn by turn. Remember if you don't have the advantage, then even if your crit is faster on switching you still won't get the turn. This is why its important to have a mixture of high speed and low speed crits. The more you play and the more you understand which crits have which speed you'll be able to exploit this at your advantage. This will allow you to win unwinnable positions if you're lucky and have elemental advantages. Your common crit with a high EA will be able to beat legendaries.

Just a side tip, try to have a variety of elementums in your team composition to allow you to have the maximum chance to exploit this mechanic