Zikura, are you talking about normal or heroic?
Are you using the sawblades and/or the superheated fire?
Either way, try glyphing touch of death, and using unglyphed fortifying brew (to increase max hp) and touch of death on the third shredder.
I kill the first shredder on heroic by using an agility pot, pooling 4 chi, and precasting chi wave while it is casting death from above. I'd suggest against rolling back as it takes up a gcd, if you tiger's lust yourself and stand just out of death from above you can get back in melee range fairly quickly.
After that, with agi pot and death from above, you kinda just gotta pray that keg smash crits. Usually I'm at about 70% crit, but I get the occasional occurrence of NOTHING critting at all during DFA.
It's very close for me, sometimes I kill the shredder before the 4th overload, but a lot of the time I get shit crits, or the dps drop horrible sawblades / no fire at all and I'll get the 4th. Doesn't help that I'm still using flex trinkets, but eh.
After that I take a 4th stack, immediately snapshot Xuen with the ~280k vengeance I get, then blow the next shredder the fuck up, because you really want to kill the second shredder as fast as possible, since that's when extra overloads are going to cause deaths.
The second shredder usually dies seconds after it does it's third overload. If you don't tank the second shredder, consider using Xuen for the first.
Other than that, you could ask for skull banner, tricks, etc. Boss damage is largely irrelevant on siegecrafter.