Since the party brings torches and lanterns into the otherwise lightless depths of the underworld, it is rare that they get a chance to surprise monsters. Doors present a rare opportunity to gain this tactical advantage.
Doors in the dungeon stick firmly in their frames - the wood warped by long ages of underground humidity. Opening such a door will often be too slow and loud to allow the party to surprise monsters that may be lurking on the other side. However, the party may attempt to breach the door in order to secure that surprise.
To breach a door, one or two party members will attempt to push or pull the door open swiftly enough for the party to rush through before the monsters get a chance to react. PCs with average strength roll a single d6, while PCs with high strength roll two dice. If any die turns up a 1 or 2, the breach has succeeded. (PCs with low strength can't contribute meaningfully to a breach attempt - they can still force a door open, but the time & effort required blows any chance of surprise.)
With a successful breach, the party can rush in ready for combat, and roll a surprise check with 2 in 6 odds. If the party wins surprise, they get a round of combat before the monsters have even a chance of reacting. Surprised monsters usually lose any chance of First Strike and similar reactive abilities.
After this, the party rolls for initiative against the monsters. With luck, the party can win initiative and take a second round of combat before the monsters can take their first. This may well allow the party to finish an encounter without taking a single wound!
To summarize: Roll 1 to 4 dice with 2/6 odds to breach the door, depending on who's helping. Then roll 2/6 on a single die to surprise the enemy, and roll initiative as usual.