An operator had a critical offshore water injection well which was not performing as expected, having an injection rate well below target. The well had a lower completion comprised of four frac sleeves in the horizontal section. A light well intervention (LWI) operation was carried out to evaluate the ball seats and valve status, any blockage that might be present, and to determine the required intervention scope going forward. A camera diagnostic run indicated the presence of debris. A subsequent LWI operation was planned, to deploy an electric line powered mechanical toolstringto mill out all four ball seats and perform a perforation operation across all zonesto generate the desired injectivity rate. Ball seats of reducing sizes had been deployed in the lower completionranging from 3.403 in. ID for the uppermost to 2.697 in. ID for the lowermost. The operator wanted the efficiency of a single run solution to execute the entire milling operation. This required a multistep mill of appropriate sizing to be designed, one that would mill all four ball seat sizes in the completionto a common maximum ID. This would then enable passage for subsequent deep penetrating or big hole perforation charges to be run. The electric line string incorporated a high level of instrumentation to provide real-time measurement and control of the criticalparameters for milling optimization. A Tractor would provide toolstring conveyance, rotational anchoring, and weight-on-bit (WOB). Tension/compression subs would provide in-situ measurements of the WOB. Arotationalsub would provide the torque. Monitoring and adjusting the torque, WOB, and rotational speed in real-time would enable an optimized rate of penetration throughout all steps across all ball seatsconfronted. Thorough pre-job tests were carried out on identical ball seats placed in a test-jig set up. Key toolstring parameters were monitored and the time required to mill through each ball seat was captured, this to determine the optimal tool parameters to use throughout the operation. All four ball seats were milled out successfully in a single run operation, with the toolstring providing both milling and back reaming, this ensuring no sharp edges remained as a result of the milling. The full milling operation was completed in a matter of hours, far below those experienced by the operator on previous operations. Upon return to surface little to no wear was found on the step mill and all mill sections were found to still be well in gauge. In effect, the step mill was used as a drift run, post milling operation, allowing the perforation runs to immediately follow. Following the subsequent perforation operation, the injectivity rate increased considerably, this contributing to increased production from the neighbouring producer wells.