Get a pin punch about the size of a pencil, then break the pin part off, just so it looks more like a pencil. The angle should not be as long as a pencil, though. Grind the end so it has not got the point.
Place the end in the opening (mud / water drain) at a little bit of an angle. Use a hammer and drive it up towards the cap (away from the shock body). Just take turns, back and forth, over and over

, until it starts to break free and move a little, then keep working both sides. Don't try to move it to far on one side, without working the other side.
This has worked for me for years.