Looks like a counter obviously. I would do it this way:
Have two variables, Combo and Timer.
For the Combo variable, every enemy you kill will increase the Combo variable by 1.
Then have the other Timer variable that is activated in this way: when an enemy dies, set Timer to 3. (This 3 is an example of how many seconds you have to keep the combo going, it can be however long you wish, but the lower the number the more difficult to keep a combo going.)
Have another event that says "if Timer is greater than 0, every 1 second, subtract 1 from Timer".
When/if the timer hits 0, have a routine (or function) that is activated that first takes your Combo variable and uses whatever algorithm you wish for that Combo number to be a multiplyer for points etc. THEN after but in the same actions, have it reset the Combo variable to 0.