This is the build I use for Loki in Arena, it also works well in Clash (I've had a couple clash games where I got over 20 kills) But mainly Arena.
I called it the clean up crew because that's how I play Loki (and how you should too). As you can see I don't mess with decoy until I am forced to get it. I put points into Vanish and Aimed Strike. And obviously my Ult when I can. My goal is to kill them before they can even react, a simple feat with Loki.
I walk the edges of the battle (close enough to get XP but not to where they focus me) and wait. Which isn't as boring as it sounds because you get plenty of chances to kill. Go in when you know you can get the kill and when you (hopefully) can escape without dying. Once you have Hydra's Lament, your next basic attack after each ability with be doing 30% more damage.
So the thing to do is this: Vanish to get close (behind for passive too) target and attack them once before using Aimed Strike, then quickly use Aimed Strike and hit them again (Thus activating and benefiting form Hydra's Lament twice in rapid succession.). You could Ult in, then use Aimed Strike and hit the enemy then use Vanish and hit them again to get Hydra's 3 times, but I prefer to save my ult for if I don't kill them in my first attempt and they try to escape, I'll stay close and wait for Aimed Strike or Vanish to cooldown then Ult in and finish the kill.
Item explanation: Focused on Damage with passives and Movement Speed (+48%)
-Ninja Tabi: Can buy right away (cost 1500 start with 1500), gives added mana and Atk speed.
-Heartseeker and Stone Cutting Sword: Damage and Movement Speed and passives
-Hydra's Lament: Passive mainly, CDR and Power though. Personally I don't see how this isn't on the recommended or popular list at shop, for Loki. It Adds 30% more damage to your next basic after an ability is cast. (I'm not sure if it activates when you use Decoy?, I don't think it does but I'm not sure and don't mess with Decoy anyway).
-Fatalis: To stay close after the intial attack, Loki's basic attack hit chain is 5 attacks, not the typical 3. This Item for me is one that can be swapped out depending on the situation. Just remember if you do swap it out you will loose +10% movement speed, and attack speed/passive. I like to replace this with Mail of Renewal and Bloodforge also, depending on the situation
-Deathbringer: Power and Crit Obviously. Expect 800-1000+ damage on one crit depending on item you chose for slot 5.
Depending on the enemy gods, you can take isolated enemies out one on one pretty quickly, and as always clean up those weak gods that decide they don't want to go back and heal, even if they have friends.
Loki is guerrilla fighter; get in, do damage, get out, and repeat.
Have Fun and Smite the Smiting Smiters