I will not go into detail, but I will help to some extent. First you might want to create a bunch of objects for your enemies, using the for next loops, then position them away from the player in random locations using the rnd command, now point them toward the player,and have them move a space, check to see if they have been killed, and if so have them get positioned away from the player, so that dead enemies will come back in waves, essentially you are using the same objects over and over again, so here is a summary of all that....
position objects
point objects at player, move objects
check if objects are dead, if so position them away from the player
Oh wait, I see that your using sprites and stuff, I don't know anything about 2D, so I am not sure how much all that above will help.
Guns arn't the problem, people are the problem, shoot all the people and guns arn't a problem anymore.