On the upper floor there's a terminal you can use to trigger explosions from the floor. Getting the boss caught in the explosions will damage it. The boss can fire diagonally up, so you have to be cautious when climbing. To make that safer, you can make it retreat to the right by charging it (walking toward) without attacking.
Before the fight starts, you can also return to the previous mine sections to defeat more enemies (they respawn over time) and gain ammo from them. Also look for hidden grenades, if you haven't collected and used them yet.
Thanks for the advice! Before you replied, I figured out that I could run up and down a corridor with a knife to harvest ammo from enemies. I also figured out that I could charge the walker, shoot it a couple of times and then retreat before it can fire back. This game has so much depth! You guys are brilliant! Easily one of the best games for the C64. So glad I bought the boxed version. Keep up the good work!