This is my personal opinion on KRF difficulties (campaigns only, excluding iron and heroic)...
under the following gameplay conditions:
① Veteran mode
② Level 10 free heroes
③ limited star upgrades (at most 5 stars are earned per stage. 65 stars are needed for full upgrades.)
④ Calling all waves early, if not instantly
01 Hammerhold: ☆
02 Sandhawk Hamlet: ★
03 Sape Oasis: ★☆
04 Dunes of Despair: ★★ (Assassin's Guild, Crossbow Fort, and Mirage are unlocked)
05 Buccaneer's Den: ★★☆ (DWAARP is unlocked)
06 Nazeru's Gates: ★★★☆ (Archmage is unlocked)
07 Crimson Valley: ★ (Knights Templar and Cronan are unlocked)
08 Snapvine Bridge: ★★ (Tribal Axethrowers is unlocked)
09 Lost Jungle: ★★☆ (Necromancer and Bruxa are unlocked)
10 Ma'qwa Urqu: ★★★ (Battle-Mecha is unlocked)
11 Temple of Saqra: ★★★☆
12 The Underpass: ★☆
13 Beresad's Lair: ★★★
14 The Dark Descent: ★★★
15 Emberspike Depths: ★★★☆
16 Port Tortuga: ★★★☆
17 Storm Atoll: ★★★★
18 The Sunken Citadel: ★★★★★
19 Bonesburg: ★★★☆
20 Desecrated Grove: ★★★★
21 Dusk Chateau: ★★★★☆
22 Darklight Depths: ★★★