As Ravius said above me, you have to use Draw Fire.
It's the only way to keep aggro off of your character. I've had my character around the corner and behind an object and then ordered my BOs to attack only to have half the enemy group ignore them and run 50 meters and round two corners just so they could shoot me.
Give Draw Fire to one of your Tactical BOs. It's the only way to prevent from being focus-fired by Cryptic's stupid ground AI.
So my main is a klingon science officer, I had an idea he'd be a support type, in the back healing and debuffing on my ground missions.
I equipped one tactical officer with a batleth, and gave him draw fire and some melee skills.
my second dps boff is the breen tac officer, and hes a debuff/ grenader.
lastly I have an engineer that buffs shields and a back up medic that removes debuffs.
Combat typically goes like this:
I set an enemy mob as the focus target
klingon tac officer runs in batleth swinging
mobs all target me and run at me and start beating me senseless, I run around in a circle until some of them target the rest of my crew and then melee/shoot the remaining 3 or so until they die.
what am I doing wrong?
Bottom line is that you aren't doing anything wrong. Except for doing the focus fire to start combat. The best way to avoid aggro is to use a an ability which will put your BO's into combat stance. Shield Generator, Tricorder Scan, Support Drone, etc. or just get your BO's close enough to the spawn and they will aggro. The ideo is to spread the aggro around amound your and your BOs. If you cur loose with a snipe or run in as point man your are to be target for tonight.
Aggro is a somewhat complicated thing with a touch of randomness in Cryptic's engine. You can minimize or maximize aggro is you know a bit how it work. I have a section on Aggro in my Bridge Office Combat Guide. The link to my guide is in my sig below.