Easy "Cry us a River" Achievement
I don't really believe you can get this achievement in solo play because you need all three enemies have only 1 HP and stay next hex to you. To get this achievement you'll need three friends. They go to you through a swamps till they have 1 HP (Better use low hp heroes to save your time). After everyone comes around a River with 1 HP, just shoot them down using River's ability.