Well there are like 2 options, tactical BO ensign slots suck. You can pick Beam overload which does some okay burst damage but you better be using EPS flow consoles because it drops your weapon power to zero, or you can go with tactical team which buffs beam and torpedo damage, although it shares a cooldown with engineering and science teams.
Torpedo spread is going to share a cooldown with HY torps and fire at will is completely useless (used to be situationaly useful in beta).
Unfortunately tactical ensign slots just really suck right now. You might be better off going with HY 1 and attack pattern beta or a beam target power or something.
Beam fire at will is useful vs one target or to clear minefields and such, it does not drain weapon power completely unlike beam overload. Target subsystems is another option as well, weapons or shields are probably the best two options.
The shared system cooldown is 20 sec btw, which means that you could use both back to back with a 5 sec pause in between if you trigger the first hy before you enter weapon range (gives you 30 sec to trigger it).