This is the build I use for Loki in Arena, Clash and also the Rare Joust where I pick him.
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.
*The skill selections may be off, I couldn't recall when you can get certain skills.
I get Vanish twice and Aimed Strike once at the start, then I alternate between the 2 abilities until I can choose my Ult, the only time I get Decoy is when I can't pick anything else.
So the thing to do is this: Vanish to get close (and get behind for Behind You! also) target and attack them once activating the bleeding damage, before using Aimed Strike, then quickly use Aimed Strike and hit them again (Thus activating and benefiting from Hydra's Lament twice in rapid succession.). You could Ult, Assassinate 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.
As you can see, I do not mess with his Decoy because to me it is a waste, Loki shouldn't worry about clearing minions, just worry about killing Gods. You can choose to get it to help with clearing minions/jungle camps easier, however you will be sacrificing early damage vs Gods.
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. Lately I have been buying Combat Boots first and just coming back to get Warrior Tabi for the added damage.
- Heartseeker and Stone Cutting Sword : Damage and Movement Speed and passives (passive of heartseeker especially with Loki's chain of attacks being 5). Sometimes I will use Masamune in place of Stone Cutting Sword, mainly in arena (or assault if you get lucky and get Loki) for its passive.
- Hydra's Lament : Passive mainly, CDR and Power help. A must have in my opinion 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).
- Deathbringer : Power and Crit Obviously. Expect 800-1000+ damage on one crit (when attacking from behind for passive)
- Bloodforge : Major power boost, Lifesteal could help sustain if they decide to fight instead of run. Passive shield helps protect you so you can escape when you chase someone too far just for a kill (I get a little aggressive in securing kills)
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 or are just slow at getting back.
Loki is guerrilla fighter so get in, do damage, get out, and repeat.
He is good at poking too, with the combo of Vanish and Aimed Strike with Hydra's Lament you can take even a tanky god down halfway in just a hit and run.
Have Fun and Smite the Smiting Smiters