Yes you must use Maya, but you don't need to animate anything. Just try this:
1. Select all your joints (in outliner, not by dragging a rectangle in viewport, otherwise it will only select the highest joint in the hierarchy!)
2. Hit "S" to set a keyframe. A red marking should now have appeared on your timeline at the bottom.
3. Export all as .dae as usual. Call your file something like Yourmonstername_attack.
4. Rename the extension of your new file from .dae to .dae_anim
5. Open your model (not the animation file mind you!) in Model Editor
6. here's where it gets a bit fuzzy for me... I've never actually done this. But looking at the original Grunt enemy, it seems to demand the following attack animations:
SwingClaws01
SwingClaws02
SwingLaunch
So add these by clicking add in the animation window, name them correctly, then for each of them click on the button next to "file" and choose your animation file.
I have no idea if this works, but it seems like the most logical thing to do. So give it a try!