To avoid laser security, take a human shield and go through the lasers and the guns won't shoot you.
Easy challenges
In the mission where you infiltrate the White Box company, after killing the guard by the front car lot, infiltrate the building. When you first see the room, you can climb up the pipe or throw an object (frag grenade, proximity mine, remote mine, sticky cam, or EMP grenade) into the room. Even on the Realistic difficulty, the most the AI can do is shout and raise their guns, provided you have upgraded each gadget to the maximum. This room provides many ways to get and complete any challenge, except for most of the Splinter Cell challenge group. Note: This can be repeated as many times as desired by reverting back to last checkpoint.
Realistic Achievement
This is a temporary fix for an existing glitch (this may be patched with future updates). If you do not unlock the achievement for completing story mode on Realistic difficulty, replay the last two levels on Realistic, and the condition should be met.
Weapons
Complete single player story and co-operative story (online or local) to unlock all the weapons and gadgets required for the "all weapons" and "all gadgets" achievements.