Given the fact that I didn't want to make investments in cards before having the meta established and stabilized, I'm currently grinding away in the lower level brackets, mostly with and against level 1 cards. So, since I'm mostly encountering low rating opponents, I figured I might as well give my views and tips for new players, on what cards should they focus on, at least for now.

These are the monsters on which you should focus upgrading, or at least having them. They are widely the most useful, and you'll use them pretty much in any deck regardless of battle mode. Of course, I'll try to recommend as low rarity as possible, because this is targeted to beginners and players who haven't invested more than an AAA game's worth.

Fire

Honestly, not much to say here. Red can actually put out a lot of team rosters, with various results. Most of all I would recommend the Phoenix (duh), but it's a Legendary, so I'm gonna say instead that Fire Demon is pretty widespread in successful teams. The Fire Demon is a very strong last position attacker, well worth its 4 mana cost, being durable and striking for 2 damage.
And secondly, maybe the Goblin Shaman, because it is always good to have Weaken when possible.

Water

Medusa and Mischevious Mermaid here. Easy picks. Magic damage + Alric = rape. Magic damage without Alric = still take out the enemy tank fast. You'll have trouble against Void (Clay Golem) but otherwise should steamroll opponents.

Earth

First one here is your tank - Stonesplitter Orc. It's the best tank of Earth from what I could tell (sorry Rexxie). It has balanced stats and the Retaliate makes it a suitable match against stronger tanks.
Then, Magi of the Forest is the one who pairs best with your orc. 2 magic damage is insane at lvl 1 battles. Protect your Magi by proper positioning, and it will take out the enemy's first position in no time.

OK, so these are the first 3 Splinters. Remember, try to think this through a level 1 perspective. Some of these cards may not scale well later, but in the low bracket even 1 extra point of damage/hit points can make all the difference in the match-up.

See you tomorrow with part 2. Cheers!