The spear is hitting the group because Defensive Systems is activating while Double Destruction is being cast. This is happening because DPS is low. The best way to fix this is to coax more dps out of the players (including the tanks, probably). If your healers have spare energy and attention, keeping the tanks dotted for the duration of the fight will make a significant difference as well.
If you're unable to eeke more damage out of your group, then you need to do uneven damage to the tanks in order to force the phase change before a second DD is cast. You'll probably run into the hard enrage, but you can cross that bridge when you come to it (and if SC is the only tank left alive, a hard enrage can be healed through for a while).
Couple ways to handle it. 1) Have the SC tank jump down and taunt, this will cause that spire to hit him instead of the DPS. 2) Get your best DPS on SC through the first 80% then switch one to FB for the final 20%. 3) Put 3 DPS on SC to start, have a ranged stand on FB and shoot through the turret (does not cause LoS issues) then move all DPS to FB when SC gets to 10% (since you want SC to die last). I would honestly suggest #3 with your best 2 DPS on FB and one of them being ranged and shooting through to SC. That way you have your best 2 DPS dealing with the 3 add side during shield phases so they can get back on the tanks that much quicker. And more time on target = more damage = less likely to get a second DD and hit enrage.
If you are still consistently getting a second DD each phase then your DPS is too low and you will likely get hard enrage around 7-10% left if you live that long and will probably wipe, or just eek out a victory with only a couple people standing. As a fight it's partially an execution but mostly a DPS check fight so if your DPS are not geared or are underperforming you will struggle with it. If you have the resources get as many of your main raid force 27 hilts and barrels, those will add 25-40 DPS per person on their own due to it pumping up the overall weapon level along with the extra stats and that could get you over the DPS hump.