FWIW I just had a look at one before last battle (W39S43), and even though the attacking creeps can sustain a lot of pain, it seems to me the towers are not choosing their targets correctly and on top of that, they're switching targets ...
https://screeps.com/a/#!/history/W39S43?t=12891485
they're hitting the dismantler creep, not the healer, which would be my preferred choice.
Mind you, this may not be enough, as if my calculations are correct that healer can sustain about 3500 dmg per tick (is that right ? 25 heal, +300% heal -70% dmg), which the towers alone can't sustain.
I'd also say the protections were not adequate ... < 1M walls in a RCL8 room ?