Speedrunners of old games where crosshairs were not already provided on the screen often mark their TV set in the center so that they always have a crosshair when shooting. Some of the first to do this referred to it as the "Illinois Cross Of Death" because it was made by taping a cross to the screen, but today it is typically done by using a marker and referred to as "the dot".

Beyond that, about the best you can probably do is grind. Figure out some levels that are particularly easy to speedgame and then grind the heck out of them. Play them over and over until they are boring and your performance is impressive. In the process you will probably get better at aiming.
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