Your main character will be your Planeswalker, and you'll be able to select one from several once you've cleared through the tutorial. Your Planeswalker will be powerful, and will be able to use abilities to help you in battle, in addition to just being a strong unit.
You should experiment with a few Planeswalkers, see which have abilities which fit with your gameplan well.
You can summon your Planeswalker three times, but if they keep dying, you might want to rethink summoning them. When your Planeswalker is on the field, you can summon units around them, instead of solely on your side of the field, which can give you an advantage.
Your cards and deck can be customised to your liking, and over time, you will want to replace the entire deck you start with, while keeping a few things in mind.
Bombs and units which kill themselves aren't the worst thing in the game, but hardly the most helpful either. The easy-to-summon smaller units are actually great to distract enemy fire, while bigger, stronger units attack them.
And if you have a particularly slow unit, it's best to wait for the Planeswalker to be close enough to your target before summoning them on that location.
One side at a time
In order to win you need to take down your opponent's primary guardian, which is behind two towers which fling damage at your units. You don't need to take down both towers, in fact, you're better off targeting a single one before moving to the main guardian.
Just overwhelm a single tower early on in the match with your Planeswalker, and any easily summonable units. Summon more units on your Planeswalker's location to bolster the forces.
Once one tower is down, you will have a much easier time of getting to the guardian - and you really don't need to aim for the other tower unless you really want to.
In the final minute of any given game, the ManaStrike mode activates, which vastly increases how fast your mana rises, allowing you to summon units onto the field much more quickly.
Once the time runs out, whichever players with the least towers will lose, so you want to protect your tower, while unleashing hell on your opponents.
If you still have Planeswalkers in that last ManaStrike moment, it's the ideal time to summon them, use their abilities, and summon as many units on them as possible. It's chaotic, but that final minute can make all of the difference.