This is the base build if you run solo queue. If you have a friend, I recommend they play Speedshot with Chicken-o-matic so that you can replace yours with Unstable Beam. You can then also opt to exchange either of the support gadgets with offensive options such as Really Cool Sticker or even Spicy Dagger if you're good at kiting and if the boss has good openings.

Generally you wanna keep your distance, use the vitamin shot to recover HP and quickly resummon your Wolf. Since the difficulty is HARD/NIGHTMARE, the Wolf won't tank for more than 2-3 hits (or slightly more if you use Life Jacket). Life Jacket also helps you endure the resummoning process. Revitalizing Mist is even more flexible as it can help you or your teammates endure the resummoning process, I don't recommend using it while your Wolf is alive as it usually gets wasted on it.

By the end of the run, you'll most definitely be the MVP as you'll tank and heal the most, while also doing at least around 100k+ damage (more if you switch out your build the way I mentioned above). This might not sound like much, but compare that to pathetic 10k-30k a typical melee type tank deals, coupled with the fact that they usually die A LOT because tanking all that damage directly is simply not sustainable. On top of all this, it's far easier to keep the boss occupied with a lot of targets (summons), rather than a single target that unlike the summons cares about NOT dying.