Hey Hey
Masteries are definitely required in this game, as you proceed into mid-game to end-game, you move from just posions more into using giant slayer(3+ hitters) and Warmaster(1 or 2 hitters) to really push your damage on the clan boss.
Don't be mistaken either, just because they are named "offensive" does it mean that it's only for attack base champions.
Offense - when you want to push more damage ( crit rate, crit damage and Giant slayer or Warmaster)
Defense - Good for improving survivability, and adding counter (honestly I very rarely use this tree )
Support - Improving Heals/Sheilds and accuracy, with skill cooldowns, increase chances to place debuffs and 30% chance to place a buff or debuff for 1 turn longer.
80 to 90% of the time I use offensives and support trees, support simply because in most cases my champions apply buffs or debuffs and I generally like to extend on these, and offensive simply due to giant slayer and Warmaster (grabbing the crit rate and crit damage on the way down.)
bare in mind yes this can be hard at first to start, but as you complete your champions it becomes a lot easier to do(and faster).
many thanks and good luck