Commander-in-Chief Trophy in Iron Sea Defenders

This will most likely be the last trophy you earn in the game. I would estimate most levels have an average of 200-300 enemies. If you complete the first 3 Campaigns, replaying some for 3 stars, you will likely have about 25,000 kills (note that there is no way to track this, so this is just a personal estimate).

There are a couple of options to grind this once you have the other trophies - you can pick a level and replay it over and over, you can go on and play the 4th Campaign, which is optional for the Admiral trophy, or you can replay earlier Campaigns.

The 4th Campaign has a lot of enemies on each Level, and completing this awards extra anchors, which you won't get replaying earlier Campaigns.

I chose to play the 4th Campaign, then went back and replayed the 2nd Campaign fully, and the trophy popped early on during a replay of the 3rd Campaign - I would therefore estimate that you will need to play a total of about 140 Levels, give or take, to reach 50,000 kills.

If you do want to just play a Level repeatedly, Levels 2-10 and 4-10 have a lot of enemies.

I have uploaded a video of 2-10 for reference.