I like to use the same rules I used in WEG's Star Wars. The battle is going on all around them, but the PC's always get the special objective (hold off the enemy, take an outpost). Depending on how they do determines the outcome of the battle. The PC's are then responsible for the lives of their comrades. If they don't hold the line, take the bunker or whatever... their comrades will die in droves instead of a few dramatic deaths. Weapons are usually kept at a max of level 8 on phasers and the equivalent on other weapons. They power drain would be a bit much. I suspect that on a futuristic battlefield the ECM and ECCM would be so intense that troops may have to resort to doing some things the old fashioned way. The fancy targeting sensor on the phaser rifle doesn't work and they have to aim the weapon manually or something like that. For large troop engagements, don't roleplay those out. The battle is just the background for whatever the PC's are doing. Create scales of success or failure. For example: The PC's have to hold a pass against the Klingons while the wounded are EVACed to hoppers. If they only hold for five minutes , a lot of the wounded are captured/killed and the PC's won't have an easy time getting out. If they hold for 10 minutes, half of the wounded get out and the PC's have to book it to make it to the last hopper or try to make a stand with the remaining wounded. If they hold for 15 minutes, 75% of the wounded make it out and the PC's can make it to a hopper fairly easily. If they can hold for 20 minutes, all of the wounded are dusted off and the PC's make it out with no problems. If they can hold for any longer, let them have reinforcements show up, or they could have tied of so many Klingon assets that the tide of the main engagement turned in the Federation's favor. Hand out promotions and commendations as appropriate.
I hope that some if this babble is useful.
"Retreat?! Hell, we just got here!", annonymous American Marine, WWI
"Gravity is a harsh mistress....", The Tick