How do I make a function that fires an event once a number is achieved? (More info in desc)

The best way to solve this is with maths (division and modulus) First, let's divide out our levels by 5, or in other words, how many groups of 5 levels do we have? If the answer is 5, we have 1 group - this will be our Unit. (I know this is obvious, bare with me). For this example, I want to be producing 7 damage. We know that our 1 group of 5 levels should be 7 damage, so lets multiply our group by 7. This goes the same with 2 groups of 5 levels (2x7 = 14 extra damage), or 3 groups (3x7 = 21 damage). So let it be known that Groups x Damage = Total extra damage. In code, it can look like this

level = 45
damage = 7
per_level = 5 --the amount of levels we need before our damage increases
groups = level / per_level
extra_damage = groups * damage --> 9 Groups x 7 damage = 63 extra damage

Problem: If we have a remainder from our damage per level (5), it will calculate the remainder into our extra_damage variable, making our system incorrect. If we ONLY want 7 damage per 5 levels, then we need to remove the remainder. To find the remainder, you use Modulus. Modulus is simply a math operation which calculates a remainder between two numbers. In our case, our first operand will be the our level and the 2nd operand being 5. Here is an example.

level = 6
remainder = level % 5 --> we have a remainder of 1 because 6 is over by 5 by 1. 5 cannot fit into 6
--if we subtract the remainder from our level, it is 5. This is true if I have 7, 8 and 9 as well.


level = 10
remainder = level % 5 --> there is no remainder, because 5 can fit into 10.
--if we subtract 0 we get 10, which is still in every 5 levels!

Okay, now to apply the two sections. Get the unit by removing the remainder and then dividing, then multiply that unit by the damage to find x!

level = 73
damage = 7
per_level = 5 --the amount of levels we need before our damage increases
remainder = level % per_level --the remainder from our level and per_level
groups = (level - remainder) / per_level --how many groups we have from this scaled down number (for every 5...)
extra_damage = groups * damage --our unit which is scaled down and multiplied by the damage we want

Less abstractly, like what I commented and related to your code, you can do this - which is the exact same thing

damage = 2
per_level = 5
player.Data.Damage = (player.Data.Level - player.Data.Level%per_level) / per_level * damage

You can think of a remainder as starting at 0 and increases by 1 each time, until the number reaches one less than the number we are dividing by. After that, the sequence repeats. I highly recommend learning it, it's fundamental and comes in a great number of uses. I'm sorry that I wasn't able to fully explain modulus, it's probably for the best. If you still don't understand, there's another solution you can do that produces the same result, although less efficient. Have a for loop for that increments every 5 levels until it hits your level, in its scope, just keep adding the up the damage to a variable, like so:

level = 17
per_level = 5
totalDamage = 0
for i=0, level, per_level do --this loop will run 3 times because per_level (5) fits into level(17) 3 times! same logic
	totalDamage = totalDamage + 2 --add our current damage by 2 each per_level
end
print( totalDamage ) --> 6

I hope this helps, and I remind you to go and learn modulus more as it is used everywhere.