This cannot be done under the easy difficulty setting.
Easy "Beat Lava Land" achievement
Defeat the Boss in Lava Land under the normal (recommended) or hard difficulty setting.
Easy "Beat Mars" achievement
Defeat the boss on Mars under the normal (recommended) or hard difficulty setting. You can jump very high and die easily. Under the normal difficulty, you will die by falling three times. You only one chance of falling under the hard difficulty setting. Note: Level 4 is a good place to farming weapon materials (gas and red stones).
Easy "Beat the Graveyard" achievement
Complete the chapter under the normal (recommended) or hard difficulty setting. Falling deaths may cause difficulty during the first four levels. The Boss in level 5 is very difficult. To make this part easier, upgrade your bombs (bomb number), bow (arrow number) and spiked sword. Throw all your bombs at the Boss. Retry if it is not hurt by the bombs. Take him down with your bow then eventually with your melee attacks.
Easy "Beat Twilight Forest" achievement
This cannot be done under the easy difficult setting. After hitting the final Boss three times, it will jump at you. Keep a good distance from it for dodging then repeat the process.
Easy "Hoarder" and "Fully Upgrade All Weapons" achievements
About 20,000 materials in total are required. Level 4 of each chapter is a good place to farm items and materials. Upgrade the bow and the number of arrows, spiked sword, and bombs first. The first two weapons are basically useless.
Easy "Necromaniac" achievement
Use falling deaths to increase the number of deaths. Three times in the normal difficulty or once in the hard difficulty. Consider using the cloud island map (which features five level portals and the home portal) or in the last chapter map where you can keep jumping and retrying.