Your punches will have no effect on this hulking zombie, so you need to knock him out with the environment. Luckily, he's not very smart, so you can easily dodge his charging by roll dodging between his legs. In the first phase, shoot explosive gel onto the electrical power boxes, then detonate them as Grundy charges at you right before he runs over the wires. He'll rage around, disabling one power box but causing the other to drop.
Now, you'll need to scan the sewer grates with Detective mode to make them viable targets, then shoot them with gel. You need to position Grundy in such a way that he will run over a sewer grate, then into the hanging electrical wire. Detonate the gel as Grundy runs over the grate, lighting him on fire; he'll trip up, causing him to run headfirst into the exposed wire. For the third and final phase, simply repeat the process 2nd phase on the remaining sewer grate/electrical box pairing.
