Might as well finish this one up while the servers are still down for maintenance. I might get around to the main characters after I finish all the mercenaries. Main characters have their own set of skills as well.
The Witch is a Staff type mercenary that comes with a variety of skills. Like all Staff type mercenaries, she has high intelligence and magic defense, but rather low amount of HP. To date she remains the only mercenary that is considered to be flying. This means that she cannot be hit by standard melee attacks, but enemies can also run past her to hit the mercenary behind her. She has a lot of MP control skills such as Mana Dragin which depletes the enemy's MP in addition to causing some damage, Mana Recharge which restores MP for a row of your own mercenaries, and Mana Trap which steadily drains MP from your enemies over time. Mana Trap is a very useful skill because it allows you to counter Brutal Will and Seth Will. Some people use this strategy in PvP so the Witch provides a counter to it. The Witch also has a very powerful area skill called Meteor Strike. It deals quite a lot of damage, but the problem is that the AP cost is very high. In skirmish maps, it covers a huge 5 x 5 area unlike other area skills that usually only hit a 3 x 3 area. The AP cost can be mitigated with the use of an Exorcist's Speed Cast, but only by a little. Nonetheless, she is good for countering enemies that rely on stun by preventing cannons from targetting her and stunning too many mercenaries at once. One strategy some Staff mains use in PvP is to have the back row with Staff main (or Oracle), Witch, and Monk in order to prevent them from all being stunned at once.
Mana Drain is a skill that depletes your opponent's MP rather than causing damage. For the most part, it is not that useful unless you are using it in conjunction with Mana Trap and an Exorcist's Mana Burn in order to quickly dry up your opponent's MP. It is useful in Auto Battle Free League because characters without MP cannot take action there. However, many mercenaries have quite a large amount of MP to burn through making it very difficult to actually carry out this strategy in most cases. It is also rather useless in PvE because you either kill enemies ridiculously fast or that enemies have extremely high damaging basic attacks anyways.
Mana Recharge is a decent skill for those who forget to equip Elixirs or Mana Potions. It restores quite a bit of MP and is great for restoring the MP of your own mercenaries that are under Brutal Will or Seth Will in order to keep them attacks. However, if you have Mana Potions or Elixirs equipped, odds are you will rarely run into this problem. Overall, this skill is not all that useful and feels like a waste. Just remember to bring those MP restoring items and this will never be a problem.
Mana Trap is a skill that steadily drains a small amount of MP from your enemies and adds it to your Witch. For the most part, it is used to counter Brutal Will and Seth Will. If an enemy uses Brutal Will or Seth Will on a mercenary affected by Mana Trap, they lose a huge chunk of MP instead. Other than that, this skill is not all that useful. It is decent if you are trying to employ a mana draining strategy in PvP, but that tends not to work due to the huge amount of MP that players have and the fact that they often have elixirs and rely on stun anyways.
Meteor Strike is a fairly high damaging skill, but it costs a lot of MP and AP to use. It is great if you are using an area attack strategy and magic is much more damaging when stacked with other skills. While it does not seem as damaging as a lot of basic attacks, keep in mind it can easily deal 2000+ to all enemies in PvP. It is also very devastating in skirmish maps where a high level Witch can easily deal 30k damage to each enemy within range. Unfortunately, it lacks the heavy damage that range mercenaries have when focusing on one target and is not good for picking of people.