The Nexus strike mission can be of two parts, one is finding The Nexus and other is killing it. In this mission, you will see Vex and Fallen fighting among themselves in Ishtar Commons. Just like for other instances, you can choose to take part in the battle or avoiding it by running past them. But if you take part in the fight, you will gain extra experience and loot.
You can go into the other zone from the side of the Commons. You can also use it to reach Darkness Zone of the Strike. When you reach the next zone, there will be Servitor with Fallen guards, it will cause three waves.
After the waves, reach the final place to face Hobgoblins, Gobins and Minotaur. Try to use cover to get out of here quickly. This way will lead you to the Nexus.
The weak spot of Sekrion, who is an Ultra Hydra, is his eye. Try to completely focus on his eye than other parts. Use Supers and Heavy Weapons targeting his eye till the health pool reaches zero and it explodes.