you should give a link to a plugin if it has any bearing on a question - and this one does.
If it were a regular battle I would simply place the change actor grafic command in a troop event, but most ABS-type scripts and plugins don't have the battlescreen.
for effects on the map I would use a common event on parallel process with a wait of at least 10 frames - and if something only rarely needs to be checked with a higher wait.
however a lot of things depend on how the game over is handled, and from your question I think you'll have to describe that in more detail - most ABS type games have only one actor and it's game over if that one dies - so something is different here. And I would place the command in a common event called on death if possible at all.