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Also different from your code, I've added a bit of delay between the enemy spawn time. You can modify it by changing ifcount's value. I changed the places of getactor stuff too, as you don't need them updated every tic. I also suggest you have a cstat 32768 in your actor code after finishing it, as you wouldn't want that actor to be seen in the game.
gamevar lo_tag 0 2 gamevar MONSTER_TILE 0 2 gamevar COUNTDOWN 0 2 eventloadactor INVASION getactor[THISACTOR].shade COUNTDOWN getactor[THISACTOR].hitag MONSTER_TILE getactor[THISACTOR].lotag lo_tag // cstat 32768 enda useractor notenemy INVASION ifvare lo_tag 0 killit else { ifcount 8 ifvarn COUNTDOWN 0 // You can add a delay for the spawn interval. For now I used ifcount. { espawnvar MONSTER_TILE subvar COUNTDOWN 1 resetcount } else killit } enda