One of the most important aspects of TacOps tactics is understanding how to fire and get out of LOS.
That means you should study the SOPs and the visibility limits on the map. You can be at the edge of woods, hidden, and have the SOP set to reverse after firing. It's also important to understand the 15-second pulses and the commands involved. Defilate, for example, is a two-edged sword because is drastically improves your survivability but you need an extra initial 15-second pulse to get out of defilate, these 15 seconds you stay in LOS.
Overall, the main killer and the major difference between good and excellent players is artillery. It is also hardest to learn when playing the AI since a lot of "intelligent" planning is involved. Invest your learning time here.
A close second is probably helicopters, attack helicopters in particular, at least if you have resupply. They can devastate enemy columns and provide intel to ruin any plan, but they also die from M16s when you fly over a lazy corpsman pair. Plus they cost enormous lethality value, losing them looks very bad on your end-game resume (the latter also applies to any on-board system capable of indirect fire). Using them to best effect does require, just like artillery, long-term planning and strict sorting out of your on priorities: do you want attrition, intel, backstop or approach slowdown? If you try to get it all, you just get purdy helo wrecks littering the landscape.