The Sentry is a very powerful node to have on a network. Capable of turning the meekest of B-coin Mines and the feeblest of Farms into nodes capable of widespread chaos, a Sentry has the potential to turn every node on the network into an offensive force that needs to be dealt with.
The only weakness of a Sentry is the time is takes to spread around the network. Preventing the spread of an antivirus - or subduing the Sentry completely - can make the difference between an easy win and a crushing defeat.
The Backdoor combo can be used to limit the spread of Sentry reinforcement, and can have a huge impact on reducing the overall strength of a network. Using Portal to gain control of a central node, then using Shuriken to nullify the antivirus before it can spread, can render entire sections of a network impotent!
Weakening a network prior to attack will also reduce the cost of programs required to hack it successfully. As mentioned previously, a b-coin saved is a b-coin earned!

Portal onto an Black ICE or Core to maximise the damage output of Shurikens, or Portal onto a Guardian to prevent them from deploying shields. For the ultimate ease in network capture however, Portal directly onto a Sentry and chop the defenses off at the root!
Disclaimer: No warranty is implied or guaranteed by these statements.