This achievement is neon impossible to do without rocket jumping. What is rocket jumping you ask? well basically if you jump and shoot a rocket underneath you it will give you a boost which means you can get out of maps or onto higher surfaces. This achievement is still hard to get, even with the rocket jump but all it requires is a bit of practice and patience.
Obviously you cant save your game (because you cant load) so just try to avoid dying =P
I have uploaded my own HD YouTube video below which will guide you through the whole process of the rocket jumping and all the way to the end boss fight.
If anyone is having trouble with the boss fight, there is a much easier way of doing it -
When you enter the cathedrals courtyard, head for the left side. You can hide there in the left corner of the cathedral's wall. The majority of the enemies Mordekai spawns should appear in the middle or the right side of the courtyard so they shouldn't bother you if you stay hugging the corner. The enemies that do spawn on the left side aren't much of a problem until Mordekai is left with a quarter of health, at this time i highly reccomend you equipt the minigun. The reason i mentioned the minigun above all others is because it can take out the Bio-Mechanoid, Majors (big red chicken lookalike) rockets quickly without them physically reaching you.
Mordekai has his own spawning patterns, he should spawn twice on the right side, twice in the center and twice on the left. I have no idea which order it goes but thats the basic pattern. (Once upon a time, he decided to spawn in the same place 4 times in a row so yeah sometimes it can be pretty random..lol)
This method may take longer then running around the map spraying the hell out of Mordekai but its a much safer approach.
*PLEASE NOTE* Due to my videos being uploaded in widescreen, the video looks relatively small in the TrueAchievement video box therefore i would suggest turning pop ups off, then clicking on the text "Serious Sam: TSE - Cathedral King Achievement" in the video box to open up a new window for YouTube where you can view the whole picture. If this doesnt work try holding down the "Ctrl" key and click on the text.
To watch the video in HD make sure you put the quality on 720p.
Anywho Good Luck Fellow Achievement Hunters =]