Virtual Training can be accessed from the pause menu and consists of 21 challenges across 4 different criteria. Free Run, Stealth Assassination, Locate and Combat. You only have to finish 3 of these challenges with a gold medal and they can be spread across the criteria if you want.
There doesn't seem to be an agreement on what challenges are easiest, so pick from the criteria you think you will be best at.
I would advise doing the stealth assassination criteria and completing the 3 "All Weapon" challenges. As these give you plenty of time and allow you to use the crossbow to take out enemies without wasting time climbing up to them.
For those of you who are stuck on them I managed to them all with about 10 - 20 seconds to spare using the methods below.
All Weapons 1
As you start switch to the crossbow and climb the red stairs in front of you onto the building.Turn around (Facing your starting position and jump across to the next building and quickly shoot the first guard, he may spot you but don't worry as long as you don't take too long you should be fine. Jump across to his building and follow it to the right. At the edge of this building shoot the guy in front of you and jump across to the roof above him, go to the right corner and shoot the guard on the roof above you and then shoot the one below you. and jump across to the next curved building and shoot the guard round the corner and climb the red bricks that he had his back to. run along the diagonal beam to the end and then jump across to next one, quickly pull your self up and shoot the guard ahead. walk to left on the beam a little and you should be able to take out the guard below, who is just to your right, and then run off the beam and to edge of the building to take out the final guard.
All Weapons 2
Stick with the Assassins blade and climb the steps to your left and then jump across the three beams and climb up to assassinate the first guard. Run to your right and climb wall infront to take out the next guard and then jump across the beams to the next, now switch to your crossbow. Take out the guard on the big building that's to the right of the four horizontal beams, then jump across to the second beam to take out the two guards infront (One above, One below). Jump the rest of the beams onto the building then turn right and jump down toward the 7th enemy. Kill him and climb up to take out the last enemy that's just above you and to the right.
All Weapons 3
Take the steps just to your right and then turn right to shoot down the first guard above you. Turn right again and jump down and shoot the guy above the red bricks and climb up to his position.turn right and shoot the guard that's to the right and above you, When he dies another guard will come along to investigate quickly shoot him.and climb the wall infront of you. Peer over the left edge and shoot the guy directly below you then shoot the next guy who is on the same side but further along. Drop of the edge and take out the next guard just below you and run across the beam to your right about a third of the way and quickly shoot the last guard.