You need a slot 1 cart to dump GBA games with GBALdr. I'm sure it would be possible for someone to write a program for dumping GBA games with a slot 2 flashcart (you might have to swap carts 10 times though), but GBALdr is specifically designed for slot 1.
The only other methods of dumping GBA games that I know of require the hardware that was used for flashing old GBA flashcarts. Either a GBA and a cable to connect it to the computer, or an external flashing device of some sort. None of that stuff is very easy to find anymore.