Xjyuk

I am creating a mob arena using command blocks and redstone. But i need to know how to be able to create different levels that can only be started by defeating the entire prior level without having multiple buttons, and if i had multiple buttons for each level the player could abuse this system by farming a easier level to get items to buy items for the next battles, not to mention i am creating a hundred levels so thats a hundred buttons and a hundred redstone lines... So i would like to have one button that you can only click after defeating all of the current wave to be able to start the next wave.

Mob drops are unreliable so i cant create a giant hopper system requiring a drops from the mobs of that wave to start the next level, sometimes you will get extra drops then you need so they can start it before the current level is complete and sometimes less, not to mention i am using mob drops of a way to buy items so that would undermine the shopping system i have in place.

I haven't played minecraft since it seems countless updates ago so all my prior knowledge of commands has died because they changed the format for most commands.

You can tag all of the mobs from a level n with the scoreboard tag "leveln". Then, you have a single command block with execute unless entity @e[tag=leveln] if score @p currentLevel matches n run setblock (some coords) minecraft:redstone_block with your button on it. This way, if the button is pressed and there are no entities with the tag leveln (they've all been killed) it will place a redstone block next to your start command block for the next level and then you chain command blocks off that one. It also checks a scoreboard to make sure you are on the correct level for additional validation. You would need to create that scoreboard when the world loads somehow, and have the last command block in the chain set it equal to the correct n. This way, if you are on level 5 and kill all mobs, your currentLevel scoreboard is on 5, and you can click the button. This spawns in mobs with tag level6, and sets your currentLevel to 6. The only possible button that will work after you kill those mobs is the one for level 7. Keep in mind this will only work for singleplayer because of the score @p part so you may need to change the selector to make it multiplayer compatible. Unfortunately, I think you will need n command blocks with buttons if you have n levels because you can't take scoreboard data out and use it as coords or something like that to only have one command block.