[This is mostly my own head!canon-y answer to the eternal question of why Helena didn't just put the barrier bubble around the bomb at the end of 3x12. I'm sure it's been done before, but I couldn't resist. Disclaimer that I don't own anything etc. etc. and warning about spoilers for the end of season 3 and character death is implied I guess]
Helena always had a contingency plan. A way out, an escape if things went badly. By now it was mostly habit, but for a long time it had been how she survived. The bronzer and the Minoan Trident had both been contingency plans once. She had learned the hard way that things rarely worked out the way she wanted them to, and she had also learned that there are some things that couldn't be planned for.
Walter Sykes was one of those things. His plan had rivaled some of her own in its sheer brilliance, and utter insanity. And he had a contingency too. A bomb in a wheelchair, guaranteeing that the Warehouse would be destroyed no matter what happened to him, and all he had to do was activate it once he was inside. Although his plan had lacked elegance, relying instead on brute force, she had to admit that it had been effective. Insane, but brilliant.
And now he was dead and she was left rushing through the Warehouse with Pete, Artie, and Myka, trying to find a way to disarm the damn thing. And that was something Helena wasn't sure that they could do. She had every confidence in the team's ability to save the day from almost anything, but not now, when they had been two steps behind Sykes from the outset and time was running out.
By the time they reached the Ovoid Quarantine Helena knew what she had to do, she had a plan. For most of her life her contingencies had been about saving herself, alleviating her own pain and anger and grief, but now something had changed. She had changed. Because she no longer cared if she survived as long as the others did. As long as Myka did. Brave, strong Myka, who had managed to reach inside of Helena's heart and make it whole again, who thought that Helena's life was too high a price for the safety of the Warehouse, who truly believed against all the evidence that Helena had been redeemed.
As the others attempted to neutralize the bomb, Helena found the control box for the containment field. One of the first things that she had done when she was reinstated to the Warehouse was to familiarize herself with the schematics of the security system, just in case. And although her intentions had been less than pure, the knowledge was proving to be useful. If the Ramadi Shackle worked the way she suspected it did, then all she needed to do was reroute some power from the main grid, reverse the polarity of the secondary transistor array, and hope.
In her peripheral vision she saw Pete attempt to force his way through the bomb's casing with a blowtorch; she wasn't surprised when the casing held and detonator continued its inexorable countdown. She freed the cables she needed from the control box and looped the excess coils around her arms as the live ends began to spark.
"And here we go," she said, almost to herself, as she brought the ends together, directing the beam they generated to a spot above the others, causing a force field to spring up around them.
Their confusing was evident as they were enclosed in the protective barrier. Myka was concerned, Artie was suspicious, and Pete was annoyed as she pulled the wires apart and sealed the barrier around them. Relief flooded her when the barrier held, and she saw comprehension dawn on each of their faces as she explained what she had done. It was the only thing she could think of, and she needed to keep Myka safe. She really wasn't this noble, but she couldn't bear to see the younger woman hurt again, not after what Helena herself and done to her. And she wasn't about to let Sykes win in a complete blowout, as it were. Maybe her time in the Janus Coin really had done her some good after all.
She saw the pain of understanding in Myka's green eyes as she realized what Helena had done for them, for her.
"Thank you," Helena said silently, the only thing she could think to say to Myka, because there were so many things they had left unsaid. And because she truly was grateful to the younger woman. For believing in her when she couldn't believe in herself. For giving her a reason to drop the trident and continue existing in this future that had let her down. For making her realize that she didn't need her contingency plans.
As the clock on the detonator counted down the last few seconds, Helena caught a scent that she hadn't smelled in over a century. Apples. Something Chaturanga had once told her meant the Warehouse liked her. And as the detonator beeped and the world became a wall of light and heat, Helena knew that she had done the right thing. She knew that the barrier would hold and that Myka and the others would live to fight another day, and that no matter what happened the world, her world, would be safe.