This unit is the biggest absorber in the set, which usually means it's pretty important, and there's nothing to hard counter it here. Doomed Mech is generally enough to justify 3rd Engineer.
Everyone's least favorite econ unit... Deadeye tends to dominate (read: ruin) any set where you would otherwise want a 3rd Engineer. In this set, the Deadeye player can hide behind a Doomed Mech while they snipe away all your Drones, and you won't be able to penetrate the absorb in time. That is, unless...
Thank Loonark, we're saved. Deadeye can only destroy units named exactly 'Drone'. Vivid Drone is not named exactly 'Drone', so if you see the enemy going for Deadeyes, you can just rush to convert all of your Drones to Vivids. Once the transition is complete, the Deadeyes are useless. Vivid Drone is a strong unit that we would want to buy at least some of regardless, so part of our economy will already be Vivids.
This unit is arguably the best attacker in the game. There aren't really any hard counters to it. But wait, there's another attacker that's also quite strong.
A powerful burst attacker, Scorchilla can be devastating in sets without good non-prompt defense. Generally you use it by only buying Scorchillas every third turn so that they all fire simultaneously, putting tremendous pressure on your opponent. It also comboes really well with...
... this monster. What you usually do is buy two Scorches, wait for them to hit one exhaust, then buy this, then buy two more Scorchillas, and soon enough you'll have some 15+ damage coming in alongside your 16 freeze. And Hellhound only has 1 HP... it looks like Scorchilla Kit might be the best strategy here. Well, except for the following unit.
Here's that preemptive defense we needed. 7 HP non-prompt, deals a damage and threatens a second, not to mention, it's partially vigilant. One damage is dealt even if you block with it. That makes this unit really good against Endotherm Kit.
So my conclusion is that the best strategy is to absorb on Doomed Mechs while you construct a massive army of Hellhounds. Be mindful of your opponent going for Deadeyes or Endotherm Kit, though. As long as you prepare properly your Hellhoud army should triumph.
One last question: Do we go high-econ? I said above that Doomed Mech is generally enough to justify 3rd Engineer, and base + Doomed Mech is indeed a 3rd Engi set. However, a large part of the reason for that is because in base + Doomed Mech you need two Blastforges for Doomed Mech, and if you're trying to rush then you're going Animus, which means you'll have BBRR to spend every turn once you get your Doomed Mech up. Spending all that on a low-economy will be difficult. Hellhound, however, fixes that problem by spending BR a piece for just 5 gold. Therefore, I think this is actually a medium-economy set.