Guidance is a powerful perk, but takes time to build up. Every 13 minutes past the first minute after taking the mastery, each player will assign a stack of guidance (up to a maximum of 10) to whoever has the lowest damage score. Much like Onslaught, guidance will only affect your hero, and will only affect your base damage.
Guidance provides the following per stack, up to a maximum of 10 stacks:
- 5% damage
- 0.1015 energy regeneration
- 2 shield armor
- 3% health
After 40 minutes, guidance will provide the same effective 20% damage as onslaught, but keep in mind that onslaught provides this 20% damage instantly, and also provides movement speed and extra shields. In most games that last longer than 1 hour, guidance is worthwhile, but guidance takes a full 2 hours to max out if everyone in the game takes guidance.
Since guidance always applies to whoever is lowest in damage, you can control who gets guidance, and stack an extra 40% damage on them within 13 minutes in a 4 player game. In a speedrun setting, it's not particularly useful except to buff a single player (such slow starters like Nova and Tychus) who do great late game DPS or are exceptional nukers.
Onslaught and Guidance bonuses are added together, up to a maximum of +70% damage, rather than multiplied together as you might expect. The Focus talent, or Fury's Metalstorm are also added, not multiplied, which means that Onslaught and Guidance are significantly less effective on Fury compared to other players.