Arthurian Legends has made this deck the low-curve aggro/tempo deck I always hoped it could be. The low curve means you can afford to cap out at 3-5 sites and maintain efficient tempo and not have to load the deck with too many minions to defend them. Try to mulligan for a Ruins or at least one site for each element, and always be applying pressure to the opponent by any and all means at your disposal. A deck this light on minions is not meant for much grinding, but it can still go the distance if you're smart about how you move your avatar around and use your resources.
Artifacts
Amulet of Niniane: This fancy trinket is an amazing bargain at a mere one mana and is one of the artifacts that allowed me to drop Daperyll Vampire from the list. Having this on your avatar early means you don't have to worry about being whittled down by magic and is a major advantage in the mirror match and against Archimago.
Ring of Morrigan: The other fancy trinket that allowed me to tighten the curve on this deck and forego Angel's Egg as well. With twenty-nine magics in the deck, this thing will be firing off quite a lot if it's on your avatar or anyone else when paired with Merlin's Staff.
Torshammar Trinket: Not so fancy, but its low cost and versatility makes it worth including a single copy. You can put it on your avatar or any minion about to make an important attack. It can be a consistent source of damage for Far East Assassin, or allow you to push a bit of extra damage every turn against an undefended site or smaller enemy minion depending on what other artifacts you've already played.
Merlin's Staff: Can turn your avatar or a minion into a dangerous attacker very quickly given that most of your spells only cost two mana. This is one of the cards that can allow Grim Reaper to weed out several enemy minions and decimate your opponent's offensive options by enabling it to survive multiple attacks.
Crown of the Victor: Best put on your avatar in most situations. However, this can go on Grim Reaper and significantly increase its longevity if it has already killed something and survived with the help of another clutch card like Grapple Shot. Has a similar effect of buffing Far East Assassin significantly with relative ease.
Screaming Skull: Another artifact that works most efficiently when on your avatar and turns it into a card advantage machine against aggro decks that play a lot of smaller minions. Alternatively, it also allows more offensive versatility by making it easier for your avatar to hit and run, attacking and killing a minion then moving back to make retaliation on the following turn harder for your opponent.
Seven-League Boots: The best movement boost mana can buy. This lets your avatar get around very easily and skirt around being intercepted, in some sense saving you a lot of potential life loss in the long run (no pun intended) without having to use up so many spells like Teleport and Blink. It's also another option for Far East Assassin and War Horse, as few minions are safe against them especially if there's also a Poisonous Dagger on the board.
Minions
Far East Assassin: Kind of works like a shadow copy of your avatar. In situations where your avatar is preoccupied, these guys can put pressure on the opponent from another angle. Their stealth ability allows them to approach close enough to pose an imminent threat, especially if you choose to arm them with an artifact like Merlin's Staff.
Grim Reaper: Should be held in hand until it has a high value target on the board to erase from your opponent's deck, ideally with War Horse or Seven League Boots to help it get around. It becomes a veritable milling machine if it manages to survive long enough to make use of Merlin's Staff and cast some direct damage spells with multiple potential victims like Firebolts.
Highland Princess: Only has three targets in the current list, but two of them (Amulet of Niniane and Ring of Morrigan) are indispensable assets to a Battlemage.
War Horse: Helps your avatar and all other minions be more aggressive by essentially giving them an extra point of movement as needed, which can be crucial to extending the life of smaller minions with potentially high impact like Grim Reaper and Far East Assassin.
Cerberus in Chains: Takes a lot of pressure off your avatar by serving as a big defender as early as turn three. You can be much more aggressive with this three-headed guardian always at your side. Putting Amulet of Niniane on it early makes it extremely difficult to kill until late game when it likely no longer matters that much. Also a good target for Grapple Shot since being dragged is not a voluntary action, and it will teleport back to your avatar once the spell resolves and your avatar moves again.
Magic
Blink: Worth a full play set for the fact that it's a cantrip and a cheap and versatile movement boost. Keeps your avatar out of harm's way after an attack and gets it around annoying sites like Free City and Troll Bridge in an instant.
Critical Strike: Functions essentially like another direct damage spell. Can make War Horse and especially Cerberus much more dangerous if you have the mana to pair it with something like Blink or Grapple Shot.
Firebolts/Magic Missiles: Simple and effective direct damage spells. Can ruin your opponent's day if cast by Grim Reaper wielding Merlin's Staff.
Blaze: Movement tech with a little damage tacked on to get through smaller minions and annoying tokens like soldiers and frogs that can otherwise slow your offensive to a crawl.
Disintegrate: Kills any unit that can be targeted regardless of its size. The range is limited but shouldn't be much of an issue given all the movement-boosting cards in the deck. Also useful if an enemy minion manages to steal an artifact and you can't afford to spend time and resources trying to retrieve it.
Grapple Shot: Amazing damage and movement tech all in one affordable package. This can enable Grim Reaper to nix multiple minions on the board or turn Cerberus into a high caliber bullet. You'll most often use it on your avatar as a kind of free attack as needed. It can also serve as a way of getting a small unit like Wisp or Princess through Gnome Hollows to finish off an avatar at death's door hiding for its life, costing you a friend but gaining you infinite style points.
Sites
Dark/Lone/Gothic Towers: A little bit of ramp to help your early tempo. Being ordinary means they won't get hosed by Atlantean Fate and leave you without Air affinity.
Dwarven Forge: Not many eligible cards yet, but invaluable for the potential to give you a turn one Poisonous Dagger or Merlin's Staff.
Funeral Pyre: Can save you some headaches against Deathspeaker if Grim Reaper doesn't show up in time to take enough victims. Also slows down Archimago if you manage to draw it early enough to matter.
Mirage: Handy tool for repositioning sites like River of Flame or Cursed Land as needed. Also good for letting you get more mileage out of sites with genesis abilities like Observatory.
Observatory: More spell selection for no mana cost. In fact, it gives you mana and Air affinity! Probably one of the most useful Air sites in the game right now and I can't find a reason not to include it.
River of Flame: Gives you a stationary Fire spellcaster that can't be destroyed as easily as most spellcaster minions. Has enough spells to work with to make it a worthwhile inclusion in the current list.
Ruins: Great to see in just about any opening hand and often worth taking a mulligan for to keep your site count low and your elemental affinity adequate.
Watchtower: Great site for protecting your side of the board from annoying trash like Infiltrate and stealth units armed with a Devil's Egg. Even better in multiples so enemy units have to fight fair while your Assassins don't.