Coughing up cash
The first thing you'll need in order to get a stronger team is earn some cash. Luckily, those of you playing the game close to launch will get plenty of cash to start with.
Even if you're jumping on a bit later, you can easily earn cash be smashing through those daily rewards and playing against others online.
Save up your cash and upgrade your units. Each unit can have cash used to upgrade them three times before you'll need something more…
In order to promote your units you'll need to spend cash to upgrade them three times, as specified above, and then you'll need cards.
Unit cards can be used to promote the units, which will give units a big power boost, in addition to a nice EXP chunk for yourself.
This will, of course, make them more formidable in battle, but will also make them more expensive to upgrade in the future. Which is why the next tip is important…
Picks of importance
Upgrading units is expensive, and as a result, you shouldn't dedicate your cash to units you're not using.
Yes, you can pretty much always find a unit that could use a boost, but it's not always worth it, unless you want the EXP rewards that badly.
My tip is to only spend cash on the units you have the cards to upgrade, and you actually use - this will ensure their power boost will be big, and will be actually useful for you.
Of course it's not just your units that level up and upgrade, it's you, too.
Levelling up your rank will give you access to more rewards, crates, units, cash, everything you need to progress through the game.
Of course you can keep levelling up by playing PVP matches against others, and upgrading your units, which will give you EXP boosts each time.