Tennak's illustration is a bit of an odd beast. He's a surprise gift that's not really a surprise, consulted a lot with the "commissioner" and also created at a particular time during my ANet partnership. Due to both of those factors, the creation of this piece is pretty thoroughly documented.
Instead of making a tutorial for any sort of technique, I'll use it as a bit of a step-by-step of the thoughts and ideas behind it.
So without further ado, in posted order:
1) The roughest thumbnail idea, just to get down the general pose. It started as a very vague mental image of maybe having the young spellbreaker use his powers to shield himself from the elements. I didn't have much of a thought about the background at this point, just maybe have a sandstorm blowing by, the current shattered by the spellbreak shield.
2) While adding some character proportions, I also gave the whole thing more than half a thought and came to the conclusion the shield idea is, lore-wise, completely bonkers. So instead I opted for a more moody piece that would also sit well with the pretty relaxed pose he ended up having and started thinking about perhaps placing it into some deserted tomb Amnoon is known for, maybe with a striking skylight.
I also had to kind of break my charr anatomy habits somewhat, since Tennak is supposed to be barely off his teenage years, so I went for a lanky body with barely noticeable hunch.
3) Researching Amnoon (and getting distracted playing for a few hours while doing so) finally started giving me some more coherent background ideas, so I once more abandoned the plans for a dusty tomb and moved somewhere a bit more spacious. It also provided a chance to add some more interesting color contrasts of yellows and blues to match the character's armor rather than just have a dusty brown place like it's 2007.
4) Having agreed on the basic idea with the "client", I started working on refining the character with some proper anatomy and details. I usually use the armor as a bit of a guide rather than copy it 1:1, since there'd be a lot of weirdly bending metal plates and such. I also like to add things that are invisible and make the characters feel more personalized. In Tennak's case, it was the amulet, Circle of Logic - a piece of gear that has no physical model in-game and only has an icon to go by. I used some stealthy ninja techniques (translation: I asked what gear does he have midway through a talk about warrior builds) to find out what accessories does Tennak use to see what can be added.
5) One of the most interesting things in the old tombs were the floors. Like... there being any. So of course i had to use that. Not only they're really nice, but their round shape also helps to break down the hard straight lines of the pyramid walls. Unfortunately, sand is still a thing, so I ended up covering most of them.
6) Refining, refining, refining. This is where it's usually difficult for me to find work balance. On one hand, I have to keep my hand loose and head empty to not drive the painting into the ground with excessive details, but on the other, I also have to be constantly "in the zone" to give the characters and places a... well... character. Little nicks in the metal, cracked wall here, damaged stucco there...
This is also where a lot of changes takes place because I figure out what does and doesn't work.
Unfortunately, this is also a piece that reminds me not all those changes are good. Flipping between steps 5 and 6, you can probably see a key loss to the piece. Following advice from fellow artists, I eradicated the tangent the wall created with Tennak's arm. It was a good advice, and it's not like the people were wrong, but I do regret going ahead with it, because I only realized too late (a year too late) what felt off about it back then and why did the hand gesture cause me so much trouble afterwards – it fundamentally changed the planned shading and eradicated a focal point that was the sun-lit sand pouring from the fingers against the dark wall.
7) Almost finishing the detailwork, I added weapons and went back to reference images from Amnoon to add various items around the place. It kind of seemed dull and unfocused, though (I didn't realize back then it was the change noted above that caused the problem), so I had to pull some tricks to add a bit of a punch in post-processing.
8) Finished piece, with added highlights, color balance, some more textures and little details here and there. It's usually a trial and error for me in this stage to find out what works.
Kreit was happy, however, so that's what counts.
Bonus moments:
B1) Wasn't entirely sure which composition works, so I flipped the whole thing back and forth a few times. First one reads the character better (the main point of focus for usual audience is the line from top left to bottom right, since that's how we're used to read). However, it goes against the light guiding the eye, which is what works better at the bottom piece. The votes from various people I asked were split pretty even on which is better.
B2) I usually like to use the overall charr models as a more of a guide than a 1:1 reference. Tennak's case is even more so, because he's supposed to be of an age impossible to simulate in Guild Wars – almost a cub still, human equivalent of later teens. So I took the basic face he has as a model and made the features a bit more cub-like. Softer edges, short teeth, comparatively large nose and ears. I also significantly toned down the iconic charr shoulder hunch and overall chest mass, since a lot of that is what animals (and people) grow into last when reaching adulthood... As a result, I get weird questions about why am I showing off a khajiit as Guild Wars art. x)
B3) OH THE FLOORS, THE FLOORS! Kidding, they were actually kind of a fun break. It required a completely different set of thinking from the rest of the painting – figuring out design and piecing it together from a few blurry, fractured screenshots, as well as trying to find layer settings that make my job easier (for example that emboss panel) requires, at least for me, a different set of creative skills to utilize and served me nicely as a palate cleanser.
Sure, I may have covered 80% of it with sand afterwards, but it served its purpose in more than one way. Sometimes, even my insanity has a system.
And when you put them all together, you get a progress gif:

Digging for these, I found a bit of a stash of old progressworks. Not planning to spam you with them out of the blue, but I may have a use for them later on, if that's something you'd like to see.