Do the players surprise the guards (4 in 6 likely): 1d6=5, so yes.
This is an unusual "two part surprise" situation. I'll run the missile group first, and then then melee group. All will have surprise.
Individual missile fire, even at normal types, is handled per fantastic combat. I.e., one throw at best FC.
Thanis (stealthy hero; THAC2 14): 1d20=7+2+1 = 10 vs AC 6 is exactly a hit. 1d6=5 hp damage. Dice for hp: d6+1 = 4; dead.
Witherun (agile hero; THAC2 14): 1d20=16+2+1 vs AC 6 is a hit. 1d6=6 hp! Dice for hp: 1d6+1 = 6; dead.
Flin (strong hero-1; THAC2 15): 1d20=19+2+1 vs AC 6 is a hit. 1d6+2=8 hp!! That's a kill without rolling for hp.
Melee combat vs normal types is handled as normal combat. I.e., one throw as a normal man per man of FC.
The players get an unanswered round of blows in the surprise segment, are attacking with surprise (+2) and from the flank (so discounting shields).
Stern (strong hero, +1 sword; 5 men @THAC2 17): 5d20=17+2+1, 8, 19, 12, 5. vs AC 7 flank. 3 hits. 3d6=6,3,1 +1 (strong)=11 hp.
Garamon (hardy hero, +1 spear; 5 men @THAC2 17): 5d20=15+2+1, 6, 18, 8, 14. +2 surprise. vs AC 7 flank. 3 hits. 3d6=5,4,4 =13 hp.
Goldwen (sagacious abbess; 2 men @THAC2 17) 2d20=16+2,9 vs AC 7 flank. 1 hit. 1d6=6 hp.
Wraislin (intellectual conjurer; 2 men @THAC2 17) 2d20=13+2,17 vs AC 7 flank. 2 hits. 2d6=4,4 =8 hp.
Total 38 hp dealt, among 5 men remaining is an average of 7.6 hp per man. Given 2-7 hp per man, they can't survive, so the ref doesn't even need to roll their hp.
Possibly, that was "loud"
What now?