Go to Ian Taggart's base. Hijack a tank located in there and shoot a few projectiles at the main base building. Once it collapses, Taggart will get away in his special tank, trying to leave the Manhattan Island. However, he will stop several times, always among other tanks. Your objective is to follow the target and destroy every single tank Taggart meets on his way.
You can try destroying these machines with your blade, artillery strikes, hijacking one and shooting the others, etc. The only thing you can't do is to approach them in a military disguise, because the supersoldiers nearby will recognize you anyway.
All in all, you'll get an information that Ian Taggart is on one of the bridges leading outside of Manhattan. Run to his tank and enter it (this doesn't differ from hijacking other machines). Alex Mercer will automatically consume Ian Taggart.
Reward: 250000 EP, 1 new event available