One thing I took full advantage of as a kid was that both games had some cheesy behavior with the sniper rifle scope implementation.
In the first game, while zoomed in with the scope you could use the D-pad to move your view a few clicks left/right/up/down to refine your aim. However, this was not really turning you, it acted more like you were a ghost side-stepping and poking his head up and down. So, you could crouch behind a box, scope in on the box, then D-pad up to look over it and shoot an enemy mob who still couldn't see your character and wouldn't engage you. Or stand just slightly behind a corner, scope in, and D-pad right to shoot around it.
The last level of the first game involves fighting a boss character who has a powerful weapon and a ton of HP, and is surrounded by his infinitely-respawning goons. There's no way kid me would've beaten it without abusing this trick, which reduces the challenge to avoiding his attacks while fighting enough lower-tier enemies to obtain their precious sniper rifle ammo.
In the second game, the D-pad moves a crosshair around a fixed viewport rather than shifting the whole viewport around. So the trick doesn't work exactly the same way. If you zoomed in staring at some obstacle right in front of you, you couldn't move the viewport to see around it like you could before.
However... when you zoomed in, it worked like you were a ghost moving forward in space, instead of narrowing your FOV from your original vantage point. So you'd instead stand about 10 feet back from a corner, zoom in past the edge, and voila: you were able to see more around the corner just as if you'd walked up next to it. Sure enough, you could slide the crosshair over and snipe the enemies now visible in your viewport despite your player character still being safely 10 feet back around the corner.
Good times. They were still great GBA games even with this exploit, it just made the (scarce) sniper rifle ammo even more valuable to the player.