Stacking Skills

The following is a study on the efficiency of similar Common Skills
when stacked on the same unit.
**Compare skills such as Mass Accel (2 Acceleration per monster in room)
and Acceleration (4 Acceleration on a single monster).**
Mass Accel is a room skill, that will cool down after 6 normal attacks.

* Having 1 Mass Accel means it will trigger once every (6 + 1) actions.
* Having 2 Mass Accel means it will trigger twice every (6 + 2) actions.
* Having 3 Mass Accel means it will trigger 3 times every (6 + 3) actions.
* Having 4 Mass Accel means it will trigger 4 times every (6 + 4) actions.

Acceleration is a single target skill, that will cool down on 2 normal attack.

* Having 1 Acceleration means it will trigger once every (2 + 1) actions.
* Having 2 Acceleration means it will trigger twice every (2 + 2) actions.
* Having 3 Acceleration means it will trigger 3 times every (2 + 3) actions.
* Having 4 Acceleration means it will trigger 4 times every (2 + 4) actions.

Consider 30 actions in total. Having 4 Mass Accel on a unit in a 3 monster room will generate:
30 / 10 x 4 x 2 x 3 = 72 Acceleration status in total.

Consider 30 actions in total again. Having 4 Acceleration in a room will generate:
30 / 6 x 4 x 4 = 80 Acceleration status in total.

So, it turns out Acceleration is better than Mass Accel after the recent cool down change in patch v1.6.0, not to mention that the ratio is even worse if the room does not maintain 3 monsters.

Having 1 Mass Accel and 3 Acceleration skills = 1 Mass Accel  + 9 Acceleration  trigger in every (6 + 1 + 9) = 16 actions, which is  (6 + 36) / 16 = 2.625 Acceleration status per action
Having 2 Mass Accel and 2 Acceleration skills = 2 Mass Accel  + 6 Acceleration  trigger in every (6 + 2 + 6) = 14 actions, which is (12 + 24) / 14 = 2.57 Acceleration status per action
Having 3 Mass Accel and 1 Acceleration skills = 3 Mass Accel  + 3 Acceleration  trigger in every (6 + 3 + 3) = 12 actions, which is (18 + 12) / 12 = 2.5 Acceleration status per action
Having 4 Mass Accel and 0 Acceleration skills = 4 Mass Accel  + 0 Acceleration  trigger in every (6 + 4) = 10 actions, which is (24) / 10 = 2.4 Acceleration status per action

Having 0 Mass Accel and 4 Acceleration skills = 12 Acceleration trigger in every (6 + 12) = 18 actions, which is 12 x 4 / 18 = 2.66 Acceleration status per action
Having 1 Mass Accel and 3 Acceleration skills = 1 Mass Accel + 9 Acceleration trigger in every (6 + 1 + 9) = 16 actions, which is (8 + 36) / 16 = 2.75 Acceleration status per action
Having 2 Mass Accel and 2 Acceleration skills = 2 Mass Accel + 6 Acceleration trigger in every (6 + 2 + 6) = 14 actions, which is (16 + 24) / 14 = 2.86 Acceleration status per action
Having 3 Mass Accel and 1 Acceleration skills = 3 Mass Accel + 3 Acceleration trigger in every (6 + 3 + 3) = 12 actions, which is ( 24 + 12) / 12 = 3 Acceleration status per action
Having 4 Mass Accel and 0 Acceleration skills = 4 Mass Accel + 0 Acceleration trigger in every (6 + 4) = 10 actions, which is 32 / 10 = 3.2 Acceleration status per action

Having 0 Mass Accel and 4 Acceleration skills = 12 Acceleration  trigger in every (6 + 12) = 18 actions, which is 12 x 4 / 18  =2.66 Acceleration status per action
Having 1 Mass Accel and 3 Acceleration skills = 1 Mass Accel  + 9 Acceleration  trigger in every (6 + 1 + 9) = 16 actions, which is  (10 + 36) / 16 = 2.87 Acceleration status per action
Having 2 Mass Accel and 2 Acceleration skills = 2 Mass Accel  + 6 Acceleration  trigger in every (6 + 2 + 6) = 14 actions, which is (20 + 24) / 14 = 3.14 Acceleration status per action
Having 3 Mass Accel and 1 Acceleration skills = 3 Mass Accel  + 3 Acceleration  trigger in every (6 + 3+ 3) = 12 actions, which is (30 + 12) / 12 = 3.5 Acceleration status per action
Having 4 Mass Accel and 0 Acceleration skills = 4 Mass Accel  + 0 Acceleration  trigger in every (6 + 4) = 10 actions, which is  40  / 10 = 4 Acceleration status per action

**Conclusion:** Mass Acceleration as well as Smoke Shell and Healing Circle are only good for rooms with 4 monsters or more. Instead pick Acceleration, Absolute Barrier(Dodge is similar to Shield) or Heal. Magic Barrier has the absolute worst ratio with its single target counterpart.