Rabbitt offers some very keen insight here.
The 'hold fire' option is one that new players often struggle with, and even veterans. When to hold, when to let loose. I've had chances at OP fire go by because I held fire too long and let armor pass by while in hold fire mode. It's timing, really. I like to hold fire as long as I can, then hit leaders, stacked units and high value targets. Stacks are the most opportune target, and if you enemy comes into firing range stacked, you can really do some damage.
Weapons like sniper rifles are really useful when you want to pin units in the open as they are coming towards you, as well as taking out leaders. In many cases, as Rabbitt astutely mentioned, don't need to kill them all, just slow them down. The more you can bog your enemy down, the better off your chances will be in the end because a weakened enemy will take longer to get to the objectives, and will not have as much punch. If you've pinned a unit with the first volley, go onto the next closest target and try to pin them next. It doesn't always make sense to hammer away at a pinned unit, especially if they get down to lower numbers. I tend to go after leaders hard because if you kill them, the units around them are vulnerable. If pinned, they are stuck.
Chosing which weapons to use when is also important.
I recommend reading some of these articles, Snowblue. They might offer some insight as well.
http://tfe4.wordpress.com/tactics-articl...-archives/