You need 1800 to get all the treasures in a character's Summon board, which is 3 tickets, 300 Gems, and 5 Armor Tokens, and 6500 to fully max their Summon board. If you purchase some Triple Exp Books from the shop using gems, and use them while grinding two characters with a powerful character for the Summon fight, you will get 450 if they aren't boosted, and 900 if they are.
Honestly, for stuff like Cosmos, I would suggest just maxing Ifrit since his Summon passives can increase your Damage by like 22%, which is very high, and try to get a few points in the rest for stats if possible. It goes a lot faster then you might think if you are prepared for it, so watch out for any powerful characters coming up or here that you can use to make it faster.
Most Summon Fights are single-target affairs, so bringing a powerful Damage dealer for that like Cloud or Tifa, and making sure to use a friend like Aranea or Setzer and replacing your weaker character with them will go a long way to making it faster.
With Setzer, just keep using Red Card, it does massive damage and delays the Summon, making it more likely you will defeat it before it does any attacks on your party.
Aranea is a little more complicated. She has a mechanic where her EX buff (that she starts with) gives her a free turn if she breaks an enemy, and her second ability, Highwind, unbreaks a broken enemy so she can hit it and break it. It also leaves the enemy with a massively powerful debuff that makes your allies do way more damage to it, so take advantage of that.
Once you use Highwimd, keep using Dragoon Drop, since it Delays Enemies by a ton when used.
They may start with a full EX when summoned but Don't Use it. it is less effective then using their abilities for the purpose of beating summons since they don' t delay.
If you keep this in mind, Summon Boards will be easy buisness! It only takes roughly half an hour to max character Summon Boards if you play your cards right outside of Synergy or Special Boosted, and only like 15 minutes for characters who are Synergy, so take your time to get used to how it works, and then pop a book and take those suckers out!
Remember, you have to first get a Summon to Lvl 20 before you can do it's Trials, and you wanna do the fight right in front of the summon, as that gives you the 150 points on it's own. You also gain Wild Points during this, so you can use those to fill in the gap for characters who haven't gotten all the way if you are tired of grinding!