If you play the mobile version ( Hitman Sniper ), the "Sniper Precision" rifle or the "ICA-19" pistol sniper are the top weapons. These allow for faster follow-up shots. A trainer can't buy you skill, but grinding daily challenges can buy you the gun.
Every NPC follows a specific loop. Spend several playthroughs simply observing their movements to find the perfect windows of opportunity for stealthy takedowns. hitman sniper challenge trainer top