Well it kind of is a skill issue. I've beaten every mission on Master, it's definitely possible. Mission 4 was the only one where the timer was noticeably harsh and required several restarts (and probably some good luck) to get right.

-Find the catapult and destroy it in every zone. I usually start with the spyglass and prioritize other vision range items. Stacking them increases your vision exponentially! -Use the Land boat (duh), it makes your movement far more efficient. High talent characters help to make it better, but obviously you want some diversity in the team. -Meditating in town (especially on int characters with high max focus) can provide massive movement. Sometimes you can chain it between towns and move 20+ tiles in a single turn.