@Math-Teacher This is a good topic to start with 
Challenge Event Strategy Reveal
It’s using the old CE format, so the only difference now I’d say is that emblems on your strong damage dealers are going to be important to kill the bosses in the end stages.
Leveled troops aren’t necessary, but they most definitely help kill the first wave of enemies (particularly in the later stages) much quicker and get to the bosses faster.
I only compete in rare, but the strategy should be similar for all tiers.
Keep in mind too that competing is very expensive with resources. Battle Items in particular. Unless you are P2P, I would only recommend competing when you have the resources, don’t be afraid to hoard.