Repeatedly select the "Try Another" option until you matched with someone who is about the same level or lower than you. After winning a match, you will get two stars until you lose. Because you can never lose a rank, attempting this against slightly tougher opponents is fine. Ignore their PvP Rank, as it does not indicated anything important. However, losses will cost gold. Note: You can pay for souls and level your minions without actually leveling your hero. Additionally, manual defense fights that are won will also count as PvP victories and also advance your rank.
Easy "Mentor" achievement
You must gain enough souls to upgrade your units. Use your gold to do this by spending gold to get keys, then units, then souls. To transform units into souls, select "units" then "disenchant".
If you have a chance to match four or more mana orbs, do so. Even if that match does not directly benefit any of your fighters, you will receive an extra turn that may become useful after the pieces have shifted. Additionally, making the match blocks your opponent from making it.
The order of your fighters is important. The first fighter in your line-up will execute physical attacks when three skulls become lined up. Also, mana goes to the first fighter in the line-up that has the corresponding rune.
Collect gold from liberated cities at regular intervals. There is a limit to how much gold a city can hold, and you will lose the overflow if it is not collected. Keep playing quests in a city to increase the amount of gold that city can collect at one time. Invade other players to earn gold. Log in every day to get gold as a loyalty reward.
Focus on matching skulls over mana. If given a choice to make a match with three or more skulls, always take it unless you can match four or more mana orbs to get an extra turn.
The "Wake the Dead" skill should be used carefully. This skill from undead warriors places three skulls on the field at random. This may not result in a match-up in your favor, and instead may benefit your opponent more. Skills that offer an extra turn in addition to some kind of attack are very useful.
Spend the Souls rewarded for winning matches to train your fighters. Although your leader levels up automatically, troops can only level up by spending Souls.
When you win an iron key, use it to chests at the "Chests" menu to get three new cards. You can also buy iron keys for gold, and magic keys (which increase your chances for a rare card) for gems, which is the game's premium currency.