Just set a variable to random(0,10) before spawning.
Then compare the variable and spawn the enemy based on that. For example in early game you can have this:
Variable is between 0-6 spawn zombie
Variable is between 6-9 spawn vampire
Variable is between 9-10 spawn goblin
And then just change that based on progress.
But there are tons of simple ways of doing it. You don't need a complex algorithm that's for sure