The few seconds the two had to resist for were some of the most intense of their lives. The shield, even held and maintained by both of them, showed new cracks with every blow of Sombra's limbs, and even more so as he started to use his magic too. Every strike echoed through their horns and into their skulls, and every second of holding the shield drained their bodies of energy. They were both sweating, panting, barely able to keep their eyes focused and their legs upright. But they were still able to see when the time was right.
Shining and Cadence moved in perfect unison, fluid in their motions despite the exhaustion, single-mindedly focused on their one goal. They both lowered their heads, brought themselves a little forward as their horns began to glow brighter, and holding as steadily as they could with their hooves against the ground they lifted their heads and fired as strongly as they could through their own shield, aiming straight for the exposed portion of flesh on Sombra's chest.
With nothing to support it anymore and Sombra's blows still falling on it, the shield around them shattered, but their twin beams of energy were enough to push the creature back and put themselves out of its range. Back, and upwards. Gritting their teeth, both Cadence and Shining focused on the ground immediately below and behind Sombra, and with a magic pulse sent through their hooves broke a portion of it and caused it to raise up into something like a short ramp. Then, they kept pushing with their magic, as it began to also dig into Sombra.
But he wasn't being pushed back quickly enough. Much worse still, though his limbs couldn't reach them, his magic could. Though dampened in part by their own magic streams as it passed through them, a sphere of energy still hit them almost in full, leaving them breathless, bruised, and nauseated. Still standing. But with Sombra charging another blast, one they wouldn't be able to withstand and they were unsure they'd even manage to survive, both of them began to lose hope. Still, they pushed on as hard as they could.
Just as it looked like Sombra was about to unleash another, even larger spell against them, the swirling sphere of darkness above his horn twisting on itself and about to be sent towards them, something happened. They couldn't tell what it had been. Maybe, if they'd had the time for it, they may have suspected their magic piercing deep enough into the creature's chest to reach its heart, if it still had one. In their conditions, they merely took the event as it came. Sombra flinched, for what reason they didn't know nor care to, and his spell crumbled apart in a crackle of hazy static.
That got them enough time. Taking one last deep breath, giving everything they still had left, Shining and Cadence joined their horns. Their magic beams entwined, twisting into a spiral, and a bubble began to spread from the joint tips of their horns before being entrapped by the double helix of their spells. There it travelled forward like a bullet, and struck Sombra's chest in full. And then it pushed.
It pushed with the strength to hurl an army across a country, back and up the sloped crystal ground and farther still as Shining and Cadence pushed behind it with all the magic they had left. Sombra's body shot up, and he screamed as he was lifted from the ground and sent sailing through the air, as the single condensed bubble of the two ponies' united powers began to dig through his flesh and bones and almost through him as a whole. And he was pushed backwards and higher still, straight into the monstrous torrent of dark energy tearing through the sky above them.