With cameras like this with no spanner holes we used to use a rubber stopper pressed down & rotated. On some cameras there may be Left hand threads, so it's righty loosey, lefty tighty.
In some cases the cap may be installed tightly enough to require more drastic means.
1)super glue a the flat section of a pair of tweezers or another metal piece to the cap. It should clean up with acetone. Can't do this with plastic it tends to melt.
2)drill a pair of spanner holes & use spanners.