Im no Devin - but here is goes
Please feel free to add what you can when you find it.
Level 1/21
Kill 8 Northern Foot Soldiers
8X12 energy (30 damage)
11xp total gain
Reward
North Foot Soldier (46/54)
*************************
Level 2/21
Kill 7 Northern Archers
7X16 energy (41 damage)
19xp total gain
Reward Northern Archer (40/72)
*************************
Level 3/21
Kill 6 Water Sentries
6X19 energy (84 damage)
18xp total gain
Reward
Water Sentry (50/86)
*************************
level 4/21
Kill 8 northern knights
8x23 energy (125 damage)
24XP total gain
Reward
Northern Knight (90/82)
*************************
Level 5/21
Kill 8 Northern Wolves
8x24 energy (200 damage)
24XP total gain
Reward
Northern Wolf (120/100)
*************************
Level 6/21
Kill 7 Northern Rangers
7x27 energy (136 damage)
21XP total gain
Reward
Northern Ranger (130/150)
*************************
Level 7/21
Kill 6 Air Senders
6x29 (300 damage)
24XP total gain
Reward
Air Sentry (170/182)
*************************
Level 8/21
Kill 7 Northern Centaurs
7x31 (323 damage)
28XP total gain
Reward
Northern Centaur (220/216)
*************************
Level 9/21
Kill 6 Northern Druids
6x43 (270 damage)
42XP total gain
Rewards
Northern Druid (250/282)
*************************
Told you im no devin... bored of all the stats already - so from here on out - requirements and rewards only
*************************
10/21
Kill 2x Northern Blue Dragons
Reward
Northern Blue Dragon (320/320)
*************************
11/21
Kill 4 x Transformed Druids
Reward
Transformed Druid (300/460)
*************************
12/21
Kill 3 x Northen War Machines
Reward
Northern War Machine (350/542)
*************************
Thanks to shadow for dets of early levels.