Focus on persistence:
* As determined as a dog with a bone.
* As relentless as a tide.
* As persistent as a mosquito.
* As unwavering as a lighthouse beam.
Focus on strength and will:
* As strong as an ox.
* As resolute as a mountain.
* As unyielding as steel.
Focus on focus and drive:
* As focused as a laser beam.
* As driven as a racehorse.
* As single-minded as a bull in a china shop.
More metaphorical:
* Their determination was like a fire burning within them.
* Their resolve was as solid as a rock.
The best simile for you will depend on the specific context and the image you want to evoke.