This seems to be quite a problem for many people who are using speedhack in NORMAL PvE fights. The mercs will dissapear and the battle gets bugged. I found something that fixes it for me, I can't say it will work for everyone yet. Simply set your cpu cores to 1.
1. Open Task manager (Ctrl+Alt+Delete)
2. Right click Atlantica.exe
3. Click set affinity
4. Uncheck every box and select only one CPU