Thermal Sunder Harrow Focus Farming | Warframe.
In this video I show how you can easily farm your daily focus rep to cap in “Elite Sanctuary Onslaught” (ESO). You could also do this in regular “Sanctuary Onslaught” (SO) but the affinity gain is a little less so that’s why I recommend just running ESO. 
What you will need to do is make sure you have a spare Gauss you can subsume to the Helminth system for his third ability “Thermal Sunder”. You also want to make sure that you have a focus school lens equipped to your warframe. The better the lens, the more focus gain. For example, I recommend either an Eidolon lens or a Lua lens.
You then want to take that ability and subsume it over Harrows one ability. Thermal Sunder will work directly with Harrows third ability “Thurible”.
What Thurible does in short is gives you a buff of constant energy regeneration when you get a kill for as long as the duration of the ability (Thurible).
When you start a run in ESO or SO, you want to first activate Harrows three, Thurible. You don’t need to let your energy completely deplete. I’ll always just count to myself to six and then I start spamming Thermal Sunder as the one ability. You want to keep an eye on the duration of your three ability that way you can recast once it runs out so you can keep spamming your one for the energy regeneration.
There’s a few things I want to note here. I am running an affinity booster. This is not needed but it definitely helps out a lot. Running a booster will allow you to gain more focus in a run so you do not have to do as many levels in ESO or SO. I am also running three ability strength Tau shards. This is also not needed, but I do recommend it. I also have tow arcanes. One for ability, strength, and the other for more duration. They’re not needed, but if you have them, I do recommend them. Especially Molt Augmented. The more ability strength a frame has the better. Also ESO and SO maps always change. Some are better than others. This build works best on maps that are smaller to when the enemy spawns are close together. Somtimes you have to run more levels because of the RNG of the maps. No way around that sadly. SO maps always change every time you do a mission but ESO maps stay the same for a week until reset. This is another reason why I recommend ESO. 
To wrap this up, this is a pretty simple build.  if you’re looking to really invest into your focus school, I highly recommend this. I really hope you guys enjoy this and it is helpful. If you have any questions, please leave me a comment. Happy playing Tenno.