(Credit to this guide goes to Gordovski/Pixues/Disciple from the official forums)
Loki has a lot of starting power (150), but lower health and shields (75), but 50 armor, the same as the other non-caster/heavy warframes.
Decoy deploys an aggro-attracting hologram. It will appear wherever your reticule is pointing at. Placement of decoy is important. Enemies will still attack you if they are close enough to you, so keep that in mind if you plan on deploying it very close to you. If cast 'on' an enemy of fellow Tenno, it will appear at their feet. If cast at a high wall, it will appear at the base of that wall. Decoy can also block your bullets, so try and deploy it so you can see the enemy's side or back. It can be damaged and can die before it expires.
Invisibility - Enemies will stand around confused, there is no way for them to detect you while invisible. Melee attacks delivered will be critical hits!!!! Devastating with the right weapons, while enemies are clustered around a decoy.
Switch teleport can be used on teammates as well as enemies, and even your decoy! You can cast the decoy up high or somewhere tricky and then switch spots.
Radial disarm destroys the ranged weapons of all enemies in it's radius. Not bosses though.
Loki is one of the fastest sprinters.
Loki's alternate helmet provides better power efficiency at the cost of armor. As far as I know Loki can get the best power efficiency of the Warframes.
Loki's powers can synergize fairly well. Invisibility is a fantastic "uh-oh" button when you need to get out a pack of infected or a wall of bullets. It is also very good for staying safe while reviving teammates. Decoy is great for luring packs of enemies around it to be neutralized by an ally's AoE attack, or to go invisible yourself and wade in with a multi-hit melee weapon specced for huge criticals (as every invis attack is a crit) and just sweep the whole group, or just dump a magazine into the whole lot of 'em. Radial disarm will cause range enemies to swarm the decoy and group together even more, infected will do that anyways because they are primarily melee. Decoy can be useful on defense missions for distracting packs of enemies, making the wave more manageable. Switch teleport can get you right into the thick of a group of distant enemies, or away, and even teleport enemies into fire, burning them (I like doing this with Shield Grineer).
Basically the way I play the Loki is using speed and misdirection. There are no offensive powers so you have to rely on shooting and melee. His melee is very potent due to invisibility
He's really fast, and you can use his speed to get in and out of combat quickly as well as navigating the mission.
Here is a build you could try. Mainly focussed on invisibilty and crit damage.
Maxing out Decoy and Invisibility might be a good idea. As it will increase the duration.
- Redirection: Loki has low shields...He could use redirection to boost his shields
- Vitality: Same for Loki's hp...boost
- Streamline - Power efficiency up to 30%...This could make the invisibility just cost 35 energy..With Loki's helmet this will go lower..don't know how much..
- Continuity: Maxed out and the duration of abilities last 30% longer.
- Flow: Maxing this out will get you +150% energy, giving Loki 375 energy...Combined with streamline and continuity = invisibility aaall day handing out crits with melee.
- If you have all 4 abilities equiped you would have an optinional slot.
For your melee:
- Organ shatter...Increases critical damage. As everything is critical while invisible...this might be good to have.
- Fury: increase atack speed
- Pressure point: increase melee damage
- Reflex coil: increase charge speed
- Killing blow: increase charge damage
You could add additional damage boosters.
- North Wind: + freeze damage
- Shocking touch: + electric damage
- Molten impact: + fire damage
Of course you have to keep track of what you can max out and how you want to devide your points.
First of all Loki has a poor dose of Shield and Health both at 75 (But he has normal Armor so don't worry) but makes up with a high pool of energy at 150 and being the fastest frame availble (Without skills and mods).
Of course at first, he would be a squishy Warframe.
First of all.
1. Decoy. At first it does not seem useful to just spawn one guy. But No, this guy saves many lives. It draws enemy fire to it, draws infested to it and more importantly, all the runners to it. Useful for making those nasty people explode. Upgrading it extends its duration.
2. Invisibily. Loki's best skill so far. Being invisible does not automatically stop the enemy from firing at you. But does help. As everyone is saying, all Melee hits become critical. Also good for running away or recharging your shield so use it to your advantage. Upgrading it extends its duration.
3. Switch Teleport. The Troll Skill of Loki. Its useful and is the most troll skill ever. This skill makes you switch places with your target (Provided he is in range). This skill is good to go into a good AoE Spot or escaping the area or trolling Speeders or getting the person you hate killed. Upgrading it extends its range.
4. Radial Disarm. Most useful against Grineer and Corpus (Who have guns). This skill forces them to melee. Nuff said. Upgrading it extends its range.
Now. Loki is a good defense Warframe if you know how to use him. You can also switch teleport bleeding allies to safe areas. Just don't put your decoy next to the cryopod and let the runners explode.
Invisibility can make a room be painted red in a moment provided your melee weapon is strong enough to wipe them out.