there's also one more trick when approaching an army: don't fully attack them; instead, maneuver so that they attack you (if the monsters have the ability to move around the map, of course).
This way, you automatically lose your first turn, regardless of the opponent, and you save your turns... and they are very important.
And also, if you're playing as a Druid (Nature), don't be lazy and cast the "Scouting" spell (or whatever it's called in your version of the game) before each turn on the adventure map. This spell removes the movement penalty.
And always have a "Vial of Endurance" on hand, just in case.
P.S.