There are (at least) two levels of tactics in any good board game. First, there are those tactics that are derived from the basic text of the rules or cards. In Summoner Wars, when you learn to put your Phoenix Elf Archer four spaces away so that the Tundra Orc Smasher can't reach you on his next turn, you've started to explore that kind of basic strategy. That is, given the basic rules, how can I leverage them to my advantage? These are called EXPLICIT tactics because the idea behind the tactic is more or less spelled out by the rules: "When attacking with this archer, you may attack cards that are up to 4 clear straight line spaces away."
Explicit tactics are obvious from the text in the rulebook and the text on the cards. Cave Goblin Fighters allow you to have up to five attacks in a turn; therefore, try to swarm the enemy. Deceivers freeze up commons they're adjacent to; therefore, put them adjacent to some commons. Malidala can avoid one wound the first time she's attacked on a turn. Therefore, use her against units with low attack values and she can live a long time. Learning these tactics is simply a matter of becoming familiar with the rules and the cards.
To take your game to the next level, you need to keep in mind IMPLICIT tactics. These are tactics that aren't obvious from the text of the rules and cards, but emerge because of the way they mess with the STRUCTURE of the game's turns. Summoner Wars turns follow a very tight structure: First Draw Cards, then Summon, then play Events and Walls, then Move, then Attack, then Build Magic. Only in that order. Imagine how different the game would be if you could move these phases around or mix them together. Without the restrictions, each turn would be a huge power play.
Instead. Summoner Wars makes you work in very tight space, having to plan carefully so that you have enough magic, are in position, etc., at the right times. However, there are cards that implicitly allow you to screw with the phase order in your turn. And although no card's sole purpose is ever to move around turn phases, it is often this hidden, implicit benefit that makes an event worth playing or a unit worth summoning.
The first time I used an IMPLICIT tactic in Summoner Wars was as the Phoenix Elves. My opponent was being aggressive with his Summoner but protected him with a two-life common between my melee unit and him. Since the turn structure doesn't allow me to attack his Common with one card, then move my other card, then attack his Summoner with that card, I couldn't finish off his Summoner that turn. That's just not how the phase order works. That is, unless you have a card that implicitly allows you to do damage before the movement phase.
I looked at the Greater Burn card in my hand and looked at the board to see who I might wound with it. Suddenly it occurred to me: I could destroy that two-life common right now with Greater Burn, before I ever moved, allowing me to attack his Summoner after all! What seemed impossible due to phase order was suddenly possible because of the implicit tactic of destroying a unit BEFORE the movement phase! Subtle but devastating.
Always keep in mind the possibility of destroying a unit before the movement phase is over. Killing a unit that was blocking your way can be surprising and horrifying to your opponent. Other events that pull this off include Burn, Assassinate, Into Darkness and Besiege the Walls. Meanwhile, units who can kill during their movement include Khan Queso, Vermin, Miti Mumway, Elephants, Torodin, and Hawk.
Destroying a unit before you're done moving is a great way to invade your opponent's space in a way he/she didn't think was possible. But it's not the only implicit tactic to be aware of. What's even more powerful than destroying a unit before moving? Summoning after placing a wall.
Memorize this short list: Summoning Surge, Mirage, Forced Summon, and Reinforcements. All four of these events allow you to summon a unit during your event phase. Usually, you can only place a Wall after summoning, so that your opponent ends his/her turn knowing for certain where you could possibly summon units from. Not so with these four events. You can place a wall anywhere on your side (likely right on the front line) and THEN Summon a unit from that Wall! This phase-warping ability makes each of these four events one of the ten best events in all of Summoner Wars. Knowing where you can summon is something that your opponent will sub-conciously lean on, and you can pull that chair right out from under them.
Always keep in mind these kinds of IMPLICIT tactics when playing Summoner Wars. The tactics aren't just the basic abilities written into the cards. The best tactics derive from understanding the structure of your turn and manipulating the order of that structure in ways your opponent doesn't expect.