Recommended Heroes: Oath Keeper
Why? You are trying to keep the thing alive for as long as possible because the killing gift (alliance chest) is garbage so just use ONE hero. STR heroes tend to do less damage than DEX or INT. It's possible you have a low level grey hero that does more or less damage here. Damage has no bearing on the loot dropped.
If you are short sighted and want to ruin the event for everybody (including yourself) then use a team such as:
Petite Devil/Trickster, Black Crow, Incinerator, Tracker, Death Archer
You'll do 2-5% per x25 hit instead of 0.1% and thus kill it 20-50 times more quickly.
Note: if it is one summoned via guild bash that is different. There is no loot for hitting there, only killing so 5 strong heroes is the strategy for that version.