making events happen at specific milestones

To test the score in multiples of 10, do if score = Round(score/10.0)*10

At this point you could have it test in the same event line if a variable doesn't equal the score, then set the variable to the score and decrease a variable associated with the time it takes for enemies to spawn.

Let's take an object and give it some alt.values. Points will serve as the score counter.

Active Alt.A = tracker
Active Alt.B = frequency
Active Alt.C = multiplier

Always
-->Set multiplier of ("Active") to ("Points")/10
(This allows us to to have an alterable value that sets itself to the score but divided by 10 in order to allow spawning every 10th point.)

tracker of ("Active") < Frequency of ("Active")
--> Add multiplier of ("Active") to tracker of ("Active")
(When the tracker is lower than the frequency rate, add the multiplier to the tracker.)

tracker of ("Active") >= Frequency of ("Active")
--> Create enemy
--> Set tracker of tracker of ("Active") to 0
(When the tracker reaches the frequency value, it creates the enemy and sets it self back to 0 and the cycle repeats.)

The higher the frequency, the longer it'll take for enemies to spawn. You'll have to adjust the frequency based on the max amount of points you expect the player to receive. Press right to add to the score and watch how it spawns faster every 10th point.

Example
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There is probably a better method but i'm in a rush so i'll hopefully post a better example later.

don't use seconds as timers, instead use variables, like this:

+Always -> Add 1 to the variable creationTimer
(if your game runs at 50 FPS that means a second has passed when the counter is = 50)

now set a variable that specifies how often enemies will be created. Based on the info above 5 seconds = 250,
so set the variable like this creationFrequency=250

When the timer is = to the creation frequency you create an enemy, and reset the timer:

+creationTimer=creationFrequency -> create enemy, creationTimer=0

If you want to create enemies more frequently change the creationFrequency variable
+EVENT -> substract 10 to creationFrequency

I think you got the idea and it's much easier to work with dynamical timers like these.

the advantage is that they are dynamical, when you use the timer, something like
+Every 5 seconds -> do stuff
and you no longer want it to be 5 seconds, you cannot change it, there are workarounds to do that that involve groups and/or variables but is messy and lengthy.

They should work the same if you don't activate frame skipping (machine independent speed) which i don't suggest you use anyways. In fact they are better as they are attached to the frame rate and not time, meaning, if the machine is too old and can't draw 50 frames a second the variable timer will slow down with it but if you use the built-in timer, 5 seconds will be "real life" 5 seconds getting your game out of synchrony. (that's if the internal timer isn't dependent on frame rate to start with) but in any case, it cannot be worse.