I have made my own video of me earning this achievement on Serious difficulty to help anyone else who still needs it because i tried the other video solution but it wasnt done on serious difficulty so i died a lot...
Overall this achievement is not too difficult but it may take a few attempts to figure out where the enemies spawn, considering kleer seem to jump on you from behind quite often i recommend scanning every corner you come across just to be sure.
To start you off you want to turn infinite ammo "ON"
To do this once you choose the level, on the difficulty selection screen press "Y" then customise the settings. Make sure the difficulty is set to "Serious".
*Note changing the settings doesnt affect achievements*
Now if you follow my video up to the second rocket jump, you have a choice to either do what i did (which was to jump over and grab the cannon) or you can carry on following the wall and you should see a pad in the middle of the grass, you can walk over that and it will automatically put you into the finish area, but you will have low health and no cannon.
I recommend getting the cannon because it makes serious difficulty 100% easier especially if you use infinite ammo but by all means you can follow the guys video in the other solution and jump back in at the final area.
If you do choose to follow my method make sure you check every single corner because the majority of the time kamikazies and kleer will spawn in them randomly. You will see me in the video sometimes checking each corner twice just to be sure (i was kinda paranoid...) Anyway once you get the cannon dont rush the next part, take it slowly, it may seem like an endless wave of kleer but fear not it does eventually end.
Once you jump over the gap to the bulls just spam the fire button with the cannonballs until you clear them all out. Once you've cleared that long stretch and you walk into the massive square area, i recommend equipting your minigun and slowly spinning around in circles spraying bullets until you see a kamakazi then focus on him, then carry on spinning, just keep repeating that and you should clear them all out with no damage taken.
When you reach the part with the demon witches, running around the side of the map is your best bet whilst shooting at them with a minigun or lazergun.
The rest should be a breeze, anyway good luck people!
There are 4 different parts for my video, i have linked them below. Now the videos are widescreen so i suggest watching them on YouTube so you can see the picture much clearer.
To watch them on YouTube simply click the title of the video (in the video screen) e.g. for the 1st video click the "(1) Serious Sam TFE: "Metropolis King" Achievement Guide" text then it should take you straight to YouTube provided you havent blocked pop ups.