What my team does for both Story and HM is we stack our two strongest DPS on Firebrand to blow it out first. 15% on story means it have only about 90k health left so if your two strongest dps can blow firebrand out quicker, they can join you as Stormcallers soft enrage is a joke.
Also, tank swapping is an ancient way of doing this fight. If you have a Vanguard/Guardian/Shadow on your team who can actively taunt while dpsing, they should taunt 5 seconds after double destruction is called on the other side. This off taunt should take the incinerate armor debuff and your main tank should pick the tank back up without having to swap at all.
How funny we do the opposite for a couple reasons.
1. Stormcaller controls the entire fight, defensive systems is triggered by Stormcallers health so you should always know when it's coming 80% 60% 40% and 20%
2. Probably a bug but we start getting random bubbles if Firebrands health is lower then Stormcallers.
What we do is get Stormcaller about 5% ahead then we have our 2 highest dps stay on it. At the last defensive systems 20% we have are two ranged jump on Stormcaller for DD then we have our two Marauders on Firebrand we burn Stormcaller to about 8% then focus everything on Firebrand until its dead then flip back to Stormcaller FTW!
Once we really got into and realized what controls the defensive systems it's become really easy for us to do. Same strat for HM. Also we keep our tanks Firebrand at about 2O'clock and Stormcaller about 10 O'clock with our DPS at 6 ish on both.