Building self-confidence and assertiveness to overcome bullies is probably a lot easier than you think.
For anyone seeking to increase their own assertiveness it is helpful to understand the typical personality and motivation of excessively dominant people, who incidentally cause the most worry to non-assertive people.
It's helpful also at this point to explain the difference between leadership with dominance: Good leadership is inclusive, developmental, and a force for what is right. Good leadership does not 'dominate' non-assertive people, it includes them and involves them. Dominance as a management style is not good in any circumstances. It is based on short-term rewards and results, mostly for the benefit of the dominant, and it fails completely to make effective use of team-members' abilities and potential.
The fact is that most excessively dominant people are usually bullies. Bullies are deep-down very insecure people. They dominate because they are too insecure to allow other people to have responsibility and influence, and this behaviour is generally conditioned from childhood for one reason or another. The dominant bullying behaviour is effectively reinforced by the response given by 'secure' and 'non-assertive' people to bullying. The bully gets his or her own way. The bullying dominant behaviour is rewarded, and so it persists.
Dominant, bullying people, usually from a very young age, become positively conditioned to bullying behaviour, because in their own terms it works. Their own terms are generally concerned with satisfying their ego and selfish drives to get their own way, to control, to achieve status (often implanted by insecure ambitious parents), to manipulate, make decisions, build empires, to collect material signs of achievement, monetary wealth, and particularly to establish protective mechanisms, such as 'yes-men' followers ('body-guards'), immunity from challenge and interference, scrutiny, judgement, etc.
Early childhood experiences play an important part in creating bullies. Bullies are victims as well as aggressors. And although it's a tough challenge for anyone on the receiving end of their behaviour they actually deserve sympathy.
N.B. Sympathy is not proposed here to be a sole or significant tactic in countering bullying. Rather, sympathy is advocated as a more constructive, stronger, alternative feeling to being fearful or intimidated.
Building self confidence and assertiveness is easy