If you have enough points left, since you can't buy the final Equipment skill, purchase the first Amber Vision skill. After Mission 1 (Akenash's Atrium), purchase the final Stealth skill and begin working on the Equipment tree (more health vials, acid, and daggers).After Mission 0 (Reminiscences), you should have enough points to purchase the first 3 Stealth skills and the Silent Landing skill under Agility.Playthrough 1 - Ghost, Mercy, & Thief Insignias on every mission (No Alerts, No Kills, All Tokens and Relics) Here's an abbreviated roadmap for those that would like to go through the game without a step-by-step guide for their first run or two. Once those are done, purchase the first two skills in the kill tree, because the latter missions will have you killing goblins, which don't count against Mercy, and the kill tree will help with that. Skills: The most important skills to purchase are the stealth tree (all of it) and the agility ability for quiet landings, and the equipment tree. I'll provide one route, but remember that if you alert a guard or don't have the timing close enough to my timing in the guide, you may have to ad-lib your path a little from time to time. The levels are very open, providing numerous routes for you to take to reach the waypoints. When you're in a safe spot, you can sit and watch their paths indefinitely to get an idea on the best timing and route to traverse an area. Most of the guards in the game are either stationary (unless they spot you), or follow a very defined patrol pattern. There are no difficulty-related achievements. This will save you tons of time replaying areas if you mess something up. If you're about to attempt to cross a room with several patrolling guards and aren't 100% confident of the path, or even if you are, SAVE YOUR GAME. If you're trying a difficult jump, SAVE YOUR GAME. If you know you haven't been spotted, and you've gotten all the collectibles and whatnot, or are working on a speedrun having gone particularly speedily, SAVE YOUR GAME.
I sometimes will point out specific spots to save, but the more often you save when you haven't made any mistakes, the less time you will spend replaying areas. I'm going to start this off by stating the most obvious, but most important, tip I can give you: SAVE YOUR GAME OFTEN.