I do like the general event format, but really? Fuck that noise. I'd strongly suggest anyone who hasn't started the event to wait and grind up your lights, then play after refreshing the game since a month is a long time and this event is apparently designed to kick you in the balls and then light you on fire.
Yes, the fact that you can't try again a battle lost does make important to take your time and level up your team before trying what might be your highest level.

For how to spend cores, I personally use them allmost only at the end of events to get the event's tanks.