Best way to navigate through the campaign missions

There is naturally a spike in difficulty between the three Tiers. Snarbolax is the final challenge of Tier 1. So now you're starting Tier 2. For future reference, the final challenge of Tier 2 is the Roarmulus Twins (but that wont be for some time). There's a saying one of my old guidies told me when I was first fumbling with Tier 2...

In Tier 1 you learn to attack
In Tier 2 you learn to defend
In Tier 3 you learn to dodge

My best advice is to stumble in screaming and see how much of your face remains after the ordeal. You're going to start re-learning the game in Tier 2 (as well as Tier 3). Enemy behaviors will begin to change and they'll start doing specialized damage. While in Tier 1 you could just tank through things, in Tier 2 you need to start using your shield. Every hit will count and you need to start minimizing incoming damage.

You could spend a few hours reading guides and listening to others... However you'll never truly learn unless you dive into it. Try not to use sparks, and don't be afraid to just restart and try again.

EDIT**

That said, make sure you have 2* and/or 3* equipment. That'll help a lot. You can usually get some of the common 2* equips off the auction house for a few thousand crowns each.

Playing on Normal saves you a little bit of time, because monsters die in 3 hits instead of 4 for example. But running through halls, breaking minerals, waiting for your teammates on party buttons, etc. take just as much time as on Elite. So the overall speedup is usually not very big. But if you simply want to finish the missions as quickly as possible, then play on Normal.

If you're interested in money, not just time, then play on Elite. You get 50% more crowns than on Normal, and I doubt that it takes 50% more time. Additionally, you supposedly have better chances at rarities.

I recommend that you play on the highest difficulty that you can, without using Sparks of Life. Stockpiling those is a good idea. You want to have at least 10 at all times, in case a friend invites you to a mission that's fun and lucrative but too hard for you.