CGSize unitSize = CGSizeMake(60, 60);
CGPoint randomPointInViewableGameScene = CGPointMake(randomX, randomY);
SKSpriteNode* enemyNode = [[SKSpriteNode alloc] initWithImageNamed:@"enemy"];
enemyNode.name = @"enemy";
enemyNode.position = randomPointInViewableGameScene;
enemyNode.size = unitSize;
[self addChild:enemyNode];
SKSpriteNode* bulletNode = [[SKSpriteNode alloc] initWithImageNamed:@"bullet"];
bulletNode.position = CGPointMake(0, 0);
bulletNode.size = CGSizeMake(40, 40);
bulletNode.name = @"bullet";
bulletNode.zPosition = 1000;
[self addChild:bulletNode];
All projectiles, or SKSpriteNodes which have the sprite name set to "bullet" will destory enemy units on contact. Enemy unit sprites are named "enemy".