The best card from this event is the attack card (Vanille or Tidus card)from the 6 card pack buyable from shop which is priced at 6 summon card.
The damage once the enemy is in break condition can be up to 30k to 40k. A must card if you want to be attacker in Multiplayer.

Tyro itself is a good card and easily attainable. You also can sell it for 2 abilites card since you can fight in MP without energy if you just join the created game. At least buy one card if hard to decide.

Ifrit card is high in attack power but a little low in break power. So it is suitable for fighting water boss like Shiva during break but if you want to reduce yellow meter, other card is better. The card do can be augmented so it is useful even in future event.

My personal opinion, max it if you can but if can't, at least buy one so you can choose to power up the card later.

Tyro/A&T (Aerith & Tifa) has a 1 off heal of 10% of your HP when you unlocked the 1st hidden abilities. Just like Moogle/Fat Chocobo/Hades and etc.

Depending which class your are planning to play(or what job you have now).
Before the FFRK event gone for good, I would recommend buy FFRK packs 1 (mage/healer). FFRK pack 2 (for warrior/ranger class).

Both have very powerful V&F (Vanille & Fang - Pack 1) and J&T (Jecht & Tidus - Pack 2) which deal crazy dmg for current content and could be augmented to 4*. Definately worth it and will bring you quite far in the future content.

Farm Tyro as much as you can with the quill/pencil you can stockpile and buy the abilities scroll to upgrade Tyro, A&T, V&F, J&T to max. If you dont have grow star to augment those to 4* just stock pile 3* Tyro and sell it for 2 ability tickets. This way you can use ability ticket boost when you have sufficient mats to 4* your 3* later when the event over.

Dont bother with the basic class, save your seed. Since you have a proper job 'Knight' ask yourself if you like playing as knight. If yes then save up summoning tix to get FFRK pack 2. It has taunt, J&T and 4WOL card which very good for breaker (if you pulled a hunter/thief/ranger) job later. FFRK pack 2 would covers job range for warrior/ranger type.

1st 1-2 ability IIRC only use about 2 scroll per level it gets more when your ability 3 and above.
Maxing J&T card would be your priority if you decided to get FFRK pack 2.

If you have a real 'mage' not the starter you may want to invest on him. Not just for the the sake of clearing chapter 3 but mage quite good in MP especially Shiva. Unless you dont have any mage job then level ur starter mage to panel 3 it should be able to clear chapter 3 with some decent decks.