I’m back with the exact calculations of mana gains from Fenrir’s special.
Post updated with findings from Mana mechanics with almost exact numbers
Assuming 8/8 of Fenrir’s special:
Gains from finishing a hero: 1.7 of a tile
Gains from each minion (speculated): 0.85 of a tile
Here’s how to easily calculate how many tiles you will need after Fenrir has fired.
First, determine base tiles to fill rate. It is 8 tiles with no mana bonus.
Now, for each killed hero/monster, subtract 1.7, and for each minion subtract 0.85. Round the resulting number up, and this is your final number.
With no mana bonus and one killed enemy without minions, it’s 8 - 1.7 = 6.3, rounds up to 7. So it would require 7 tiles to fill the mana. If you kill a minion too, this will become 8 - 1.7 - 0.85 = 5.45, rounds up to 6 tiles.
With level 29 mana troop (+15%) and class bonus (+2%), to completely fill mana it would require (8 / (1 + 15% + 2%) = 6.84 tiles. So, after killing 1 enemy without minions, it would be 6.84 - 1.7 = 5.14 tiles, rounded up to 6. If one hero and 3 minions gets killed, it would be 6.84 - 1.7 - 3*0.85 = 2.59, rounded up to 3.
Which troop to use with Fenrir?
This assumes Fenrir special level 8, and that Fenrir kills a hero without minion (so 1.7 of a tile)
Without mana troops, Fenrir will charge in 7 tiles after a kill
Any mana troop (even level 1) will bring the number down to 6. Level 29 mana troop will still need 6 tiles to charge.
Class bonus (+2%) doesn’t change the number of tiles to charge, it’s still 7 tiles without mana troop and 6 with mana troops.
Spirit link (+4% mana) gives no benefit for any mana troop, but lowers the number of tiles to 6 if you don’t use mana troop.
Ariel’s buff (+24%) brings the number to 5, for ANY troop, mana or without mana.
Ariel’s buff + class bonus (+26%) shaves the number to 4 for level 29 mana troop only.
Max mana speed is Ariel’s buff + class bonus + spirit link bonus (+30%). With this bonus and mana troop level 17 or higher, Fenrir will charge in 4 tiles after a kill. Otherwise, it will require 5 tiles.
So, in conclusion, any mana troop will make Fenrir Very Fast after a kill, charging in 6 tiles.