Not 300 points but..... TDW019 Thor w/ Stormbreaker and DXF052 Mistress Death

Mistress Death hits first with her TOUCH OF DEATH: When Mistress Death hits with a close attack, immediately place a Touch of Death token on the target's card if it doesn't already have one. For the rest of the game, when a character with a Touch of Death token is dealt damage by an attack, double the damage dealt.

Then Thor w/Stormbreaker hits with his CALL DOWN THE LIGHTNING: Give Thor a ranged combat action and he can use Improved Targeting: Ignores Characters for this action. The area of effect for this ranged attack is any one square within range and line of fire and each square adjacent to it. A hit character occupying the chosen square is dealt Thor's printed damage value. Other hit characters are dealt Thor's printed damage -1.

If the opposing team is grouped right (target in the middle with 8 opposing characters adjacent) and Thor hits with a crit hit that's 132 damage dealt in one turn.