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[gabiru]

to kill the level 7 boss first I jump on the box on the left side of the screen (or right? perhaps might work too) and shoot toward the opposite end of the screen (top right in my case) until both side weapons of the Skull are destroyed. Then I change weapon to homing missile And unload the thing at the same time trying to avoid from being crushed by the skull when it loses the flame thrower.
To kill the last boss on level 21 is more or less the same strategy.

I had some problem with the joystick at first but I solved it by editing the joystick section on the dosbox configuration file to auto and everything else to false. It sorted out Tyrian 2k as well!
Another good idea (still on the conf file) might be to change all the sound sampling rates to 16000 and change the first priority to highest.