Base game + the sandbox download. You gotta extract and merge the files from the .zip folder with those of the Data folder of the base game. Your computer shouldn't ask you to overwrite anything.
I'm pretty sure the Sandbox and Altform mods are not compatible with each other as they both change the base game considerably. I would recommend using Debug along with Altforms if you want to get a specific mon or something like that :)
Hello! I got to the battle in Beryl against Taka and ZEL, but as it is about to start I get this error and the game crashes
Is there any way I can solve this?