Troop Attack stat is determined by:
- How leveled the troop is (via souls)
- How ascended the troop is (via ascension)
- How many +Attack kingdoms you’ve raised to level 10 (via gold)
- How many level-10 +Attack kingdoms you’ve raised to power level 5 (via maxing troops)
- What +Attack team bonuses a given team has (certain types/kingdoms)
- What difficulty you are playing on (for enemy stats)
Troop skull damage is based on the displayed Attack stat, which is modified by:
- What +Attack buffs a troop has received during the course of the fight
- What -Attack debuffs they’ve received in the course of the fight
A given attack is further modified by:
- Whether the attacking troop has a “slayer” attribute against the type of the target troop (x2 skull damage)
- Whether the attacking troop has status-effect boosting traits (like Pyromania) that give it x2 or x3 skull damage
- Whether the target troop has Hunter’s Mark (x2 skull damage)
- Whether the target troop has a skull damage mitigation trait (Armored = -25%, Stoneskin = -50%, Granite Skin = -75% skull damage)
In the case of Mist Stalker, many of the above do not apply, so my immediate guess would be, you were up against a fully-ascended opponent with all the +damage kingdoms unlocked.