You can look at what you're doing to the Automated Shredder by going to expression editor & entering the following string:
Code:
(sourceName = "Sanguiss" and targetName = "Automated Shredder") or (SourceName = "Automated Shredder" and spell = "Death From Above")
So that's showing everything you do to the automated shredders, and also showing when the shredder leaps + is affected by the debuff.
I routinely tank/kill the shredders on h-siege, and I have a warrior tank, but I haven't done h-siege with a warrior. That said...
1) Sometimes you get three on-the-GCD hits in while the shredder's got the debuff & sometimes you only get two
2) You're not always getting a 3 stack of devastates up prior to the death from above
3) Always pool damage for Death from Above. I'm seeing windows where your three attacks after DfA are Dragon's Roar -> Shield Slam -> Devastate, I strongly suspect you'd be best to pool a Revenge. Also ensure you can spam heroic strike through the whole debuff window
4) If you're struggling to kill it, be careful you don't clip Mines or Siegecrafter with Dragon Roar (due to reducing damage via aoe splitting)
5) If all else fails, you can get a range to swap & toss a couple of shots into the Shredder immediately after the DfA, while it's got the debuff up. Depending on how you handle stack swaps, the second time you're lower on stacks & have to kill a Shredder can be a little awkward until you get the hang of it & a quick hit from a dps or two can help you through
6) Yes, more vengeance does help. A lot of your variation in hit sizes is
probably due to that. If you log with advanced combat logging & upload to warcraftlogs, you'll be able to track your Attack Power levels to judge what's happening there (in the Resources tab). I don't think WoL has put in anything that'll tell you vengeance.