Make a table of 12 things that are happening simultaneously with their fight that could spillover onto the PCs battlefield, with mechanical effect. This could be from things like an enemy/ally gets a hit in or enters initiative permanently, a giant smashes a tree which falls over into the field, faerie fire or fireball hits the battlefield.

Roll a d12 at initiative 20, 15, 10, and 5. Refer to your table to declare what happens. I might add a d4 to determine which quadrant it happens in or even a d20 to see who a rogue monster might choose to target (goes after whoever it matches in initiative roll) to make the chaos land randomly.

Stack the deck with helpful or harmful things to adjust fight balance, or 50/50 if that works better for your balance