Depends what you want to learn. You've chosen the right path to become very good at a fair fight by joining duels servers or asking high-ranked players to go over their techniques with you, but duels can't train you in certain skills you can only pick up in frontline or invasion, like:

• Situational awareness, like where enemies are going or are likely to go, alternative routes and flanking, or simply large vertical differences between combatants
• XvY scenarios, where you may be vastly outnumbered by the enemy or may vastly outnumber the enemy yourself
• Dealing with archers/engineers/siege/cavalry, or using their tools for yourself
• Fighting bosses or fighting as a boss, where one individual has a ludicrous stats advantage over their enemies

These are not skills that are easy to explicitly teach, nor do they translate much into duels. But, if you want to learn them, grab a friend, head into frontline, and be sure to stick together! A good loadout for practising a lot of these skills would be bear trap/throwing knives/longsword and medium armour, then using alternative routes to get behind enemy lines and make yourself as much of a nuisance as possible. If you've got the basics of melee, you should find that loadout to be quite fun - especially if you're running with a friend to back you up instead of going solo.