I agree with @Demian Brecht.
It is really important to not get caught up in semantics.
What is really important is that you flesh out any design snags that
you have yet to encounter.
In other words:
The value of UML is not just to write some precise model so that the world can understand your design, but it is a tool, so that... you can understand your design, then share with your colleagues.
So, just keep drawing. Ask the questions: "Does this accurately represent the Current State, or Future State?", "Are we leaving anything out?", "Does this pass the practicality test?".
With practice, it is possible to envision diagrams before fully understanding why they represent higher-orders of complexity. Comparing any diagram to reality can then reveal unexpected oversights or optimizations (hidden-fruit).
In all cases: it really doesn't matter if the arrow has a crows-foot pointing in the right direction or not. What matters is that you are fleshing out the design through a visual process, so as that aspects of the design challenge are not hidden from your awareness at development time (time-bomb).
To directly answer your question on Activity Diagram:
What you need to do is to find a way to make your activity diagram meaningful.
If you cannot see a way to fit the enemies into the UML prototype that you are looking at, consider how it may be adjusted so that they can fit in. Do not feel limited by tradition. The goal is to visualize the design of enemies on paper.
If it is meaningful, it is valuable - and can be re-used later. If it
is meaningless, it is not valuable - ever.
So just put what you know down, and start to add on to it. When you realize that something is left out, figure out how to incorporate it visually into your diagram. This will lead you into new realms of awareness of your design that you did not originally conceive. Finding this awareness is the point of improving your designs. Only then do you finalize the diagram into a standard way that others can understand. Only then, is the Unified aspect of modeling applied. In the interim, it is up to you to discover what is unknown through iterative diagramming.
Hope that helps.