1) Varies based on mega boss. For most, a good % to aim for is 7~8%. Go for 10% if you want to be safe.
2) Level your strongest card, preferably a ++. Don't cross elements (ex. using fire to boost water).
3) Spend em.
4) Don't bother. Just evolve and focus on boosting the ++ versions of cards. Later on, you can try maxing stats on SRs and SSRs (currently unreleased) when you have tons of cards/stamina/time to burn.
5) Ideally, use gold lottos on items and you won't need to enhance. Short of that, grind for cards to boost your mains. Progress in quests for higher rank mega bosses and better loot to sell.