... playing at least one card that has either the Sneak, Snipe, or Opportunity ability.

I guess that's anyway the case in the majority of the games, so I wouldn't really need to focus on that focus. :)

For example, if a player has a Life element focus, then they will still earn focus points based on the rshares earned from a battle win even if they use a different element ...

That means it's even more profitable to just focus on playing as successful as possible, even if neglecting the current focus.

... but if they choose the Life element and use Life cards in the battle, then they will receive bonus focus points above the amount of rshares earned for each Life card they used.

I wonder why the bonus points (also for example for gold foil or alpha cards) are calculated according to the absolute number of cards and not the percentage of these types of cards in the team?
If I use for instance a gold foild "Scarred Llama Mage" plus a gold foil "Kron the Undying", and that's my whole team, then that means 100 % of my used cards are gold foil cards. If my opponent uses altogether 7 cards (1 summoner + 6 monsters) where 3 of these cards are gold foil cards they have a potentially higher gold foild bonus, even if less than 50 % of their team are gold foil cards.