In my experience, sometimes the Borg adapt to your second shot. I have also been told that if everyone is using the same damage type weapon the Borg will adapt quickly. For this reason, I try to have weapons of several damage types in my inventory so I can using something different than my other teammates are using. I've found this helps me get more shots on target that count.
Also, depending on the mission, I use a Lirpa or Bat'leth as my secondary weapon. Borg don't adapt to melee weapons, and sometimes you can't back up any further as your remodulate weapons.
I've seen engineers use their barrier ability to protect health generators. This seems to work well, as the borg almost always target turrets and generators first. I suspect a similar tactic could be used to filter the borg through a mined passageway, or a to an area with a Transphasic bomb in it
Also of note are kit choices. As a science officer I always carry two Very Rare Mk X's: Medic and Physicist, and am carefully specced to make use of both.
Healing is a generally useful skill in any MMO, but I backup with the Physicist for times when there's another Medic or I'm all alone in the switches room for Khitomer Accord. The Physicist is especially effective as it features a snare and three damage abilities all of which are a type that the Borg cannot adapt to.
As for the other two classes, I'm not sure what kits are optimal for them, but I hear Fire Team is pretty key for Tactical. Not sure about Engineering.
Bare minimum you can pick up Rare Mk IX kits from Ensign Lessa for a little over 10,000 EC, also called a "pittance". I got my Very Rare Mk X's off the exchange for varying prices.
I wouldn't rely all too much on the combat triage if i were u when playing an engi, take it from me when it comes to engis on ground stfs combat triage just doesnt cut it if u go all out on the dmg department area thus the need for the large hypos they save ure hide so i would throw in those hypos instead of using the horta.
but then again what ever works for u m8 just my take on it when it comes to engis lol ^_^
Finish the main plot, pick up the Jem'Hadar set in the Cardassian Struggle and some high-end Antiproton guns from the Undine missions (as discussed above).
EDIT: As for kits, Operative is reasonable for Tacticals, I believe.
If you get it at a high enough level, the anti proton weapon for the Devidians is pretty good against borg - if you use the chain fire mode against the borg in the back of a pack then you do flanking damage to those in front of it! Very cool and can take most of a pack of borg down to low health without needing a remodulation