Sylph doesnt draw cards but i guess with yuna, you can just w8 for his turn and when he declares an attack then play sylph on a strong dulled forward and then tap yuna for a card, but yeah always for defensive plays. It is one of few tools which you can surprise your opponent.

Also with brother but this really depends on how the game looks like. If you have a wakka in your hand search for another wakka that means you can use status reels (polymorph) on his forwards and then kill their strongest character with any forward. Ofc you can search for another wakka while wakka is in play but that will just spoil your plans but might still be the best play.

With tidus 6 cost i mostly sacrifice yuna for 2 cards to just keep the tempo and also tidus has brave while yuna is in play. I only attack if i know i can most certainly keep him alive since he is your strongest defensive character.

In early game you can take 1-2 dmg if you can build up your backups you will win later on that. Basicly every 2 backups lets you keep 1 card in the hand.