You need to make sure you trade well in battles. Make sure your rally points are not trickling units into groups of Mantis who have got across the map faster and got the numerical advantage. It's general advise for all factions but against Cybran all the more important as it can snowball that much faster. You also need to have good Intel and play zoomed out so you can intercept run bys. I think playing Cybran is more forgiving at T1 because you can react later and Mantis speed will help out. You can also finish facs which are half built (cos the engineer building them died) using any nearby mantis. This makes it easier to expand quickly.