Tchakka never resisted Stun when I casted it at any point during the battle (granted my Stun setup is about as good as it can be, but still it was 0 resists). Considering he's water-based and likely least resistant to Thunder, I doubt you have any resists on him aside from some very infrequent random ones.
Was QD spam when I did it also. Colonization was 35% at the time and Thunder V dealt about 650-700 damage as SCH (2900-3000 typical without storm so I think he has some extra MDT- or MDB as well, assuming overall DT is directly aligned with colonization rate). Normal damage should be fine once colonization rates are at or near cap, but you're still gonna want to keep people out of range of this guy most of the time.