Use Darion or Velana. Note: Make sure the Hero does not get taken out or the game will end.
Darion can cause damage on two spaces in front of him. Position an enemy between an ally with low health and Darion then use his normal attack.
Velana deals area of effect damage to all spaces around her target with her normal attacks. Have an ally with low health get next to an enemy then hit the enemy with Velanas bomb.
Easy "The Cause" Achievement
Ring runes are green. One can be purchased at any rune shop. The ring slot does not require an upgrade item, as weapon and armor slots. Go to a blacksmith and equip a ring rune to each of your mercenaries.
Easy "The Contents" Achievement
New mercenaries are unlocked as you progress. To view the requirements to unlock archetypes for the various mercenaries, go to an inn and select the "Mercenaries" option. When viewing a mercenary to hire, three tabs labeled "I", "II" and "III" will appear under their portrait. Select each tab to view the requirements to unlock that archetype.
Easy "The Corruptible man" Achievement
Occasionally a bounty hunter will appear in place of an area Boss. Speak with him, and he will tell you that he wants one of your mercenaries. Accept his request and he will take that mercenary with him.
Easy "The Detour" Achievement
The Undead Undertakings DLC is required to access the Necropolis, however you can still see the portal without it. The portal to Necropolis appears in the bottom part of Area two.
Easy "The Hero" Achievement
You have the opportunity to unlock this after unlocking the hidden weapons of all the other mercenaries. Go to Necropolis after step 2 just like you did to unlock the weapons for the other mercs, and the story for it will trigger. You also unlock the achievement for defeating the vampire with this one.
Easy "The Incorruptible" Achievement
Occasionally a bounty hunter will appear in place of an area Boss. Speak with him, and he will tell you that he wants one of your mercenaries. Decline his request and defeat him in the subsequent fight.
Easy "The Loan" Achievement
The Banker may occasionally appear at the start of step 3. When offered a loan of 15 gold, accept it. The Banker will appear again immediately prior to the final Boss. He will ask you to repay 25 gold instead of the original 15. Pay back the gold.
Easy "The Treachery" Achievement
The Banker may occasionally appear at the start of step 3. When offered a loan of 15 gold, accept it. The Banker will appear again immediately prior to the final Boss. He will ask you to repay 25 gold instead of the original 15. Refuse and he will attack you. Win the fight.
Various Steam Achievements
Complete the indicated task to unlock the achievement.
Achievement
How to unlock
The Accident
One mercenary knocks out another with collateral damage.
The Armor
Reach the maximum level of a piece of armor.
The Barbarian
Triumph over the Barbarian Chief.
The Best
Triumph over your Nemesis.
The Burglar
Lock pick 15 chests.
The Contents
Unlock all mercenaries and their archetypes.
The Deal
Buy a rune from the Thief Master.
The End
Finish the game in less than 18 days.
The Expert
Have a mercenary reach level 10.
The Explorer
Find and interact with all 4 legendary locations.
The Hardship
Finish the game in normal mode.
The Hero
Unlock the hero's secret item.
The Insomniac
Finish the game without using a campfire.
The Knight
Triumph over the Black Knight.
The Party
Have a team of 5 mercenaries in one party.
The Philanthropist
Complete all co-op quests in the game at least once.