The Wildcat Engine coop event will last only for 3 days and this combined with the fact that you get 3 chances a day to challenge the boss, means that the amount of currency you can obtain is pretty limited and you have to choose wisely what to get from the shop.
With 9 runs and 1200 Broken Cores (the event currency) obtained per run once you reach the max damage tier (3.6m damage is required to reach it) the max amount of Broken Cores you can obtain is 10800.
Since the leaderboards are not present in this event the shop rewards are the only rewards you will be able to obtain.
Alteisen MK. VI is the boss you have to kill, and it's the train you for sure fought a lot in the Interception D battle.
Honestly, the boss is pretty easy, and with the drastically lowered damage requirements compared to the test iteration of this event, as long as you get at least 1 character of each Burst type, you should be able to reach the last reward tier. Just make sure to kill the launchers and turrets and you will be able to safely DPS the boss down.
Still, while you can join the event with random people, be aware that it's risky as people might just pick a character they love and ignore the setup.
What to buy?
Tim's made a nice evaluation of what to buy in this video, so give it a watch!
Overall the Credits or Battle Date Sets are the best short-term value in the shop, but for which ones to buy, you have to do some calculations - as what gives you more value depends on the level of your Outpost. Tim explains it well in his video.
Still, the Core Dust is what you will need so much, later on, starting from level 100, so if you want to look more long-term at buying things in the shop, get it instead.
Good luck all!