This achievement, along with 13 others, can be knocked out easily at the first encounter with human enemies in the Lost Tunnel mission. Simply keep reloading the checkpoint if you wish to get this plethora of achievements out of the way. I realize that a lot of them, the weapon kills ones specifically, will come easily when doing your Spartan mode playthrough, but I like to get easy stuff like this out of the way so that I can focus on collectibles and achievements that require a full playthrough or specific areas to obtain.
The video below shows this checkpoint farming in action, unlocking most of these achievements. For Tonic Man specifically, just make sure that when you're 'going loud' getting pistol and assault rifle kills, that you keep tapping
to use your medkits while the enemies are shooting at you. Or you can just keep reloading the checkpoint like the others and run out and get their attention to spam your 4 medkits while they shoot at you.