Serra, which has won two straight CIF championships in different divisions, will meet top-seeded Windward or fourth-seeded St. Bernard in the championship on a day and site to be determined. The Cavaliers improved to 26-3.
Serra's smothering defense and numerous counter-attacks off steals played a huge role in the Guards' demise.
"We love to play great defense and then get out there with our up-tempo offense," Serra Coach Mckinsey Hadley said. "It just something that suits us well and we practice it a lot.
"Even when they came back at us with a run, we would make that tough stop and take it from there. It's about tough defense and doing it with constant pressure."
After the Guards, who won the Santa Cruz League championship before posting playoff wins against Louisville, Pasadena Poly and third-seeded Chaminade, took a 6-5 lead with 5:50 left in the first quarter, Serra used a 16-5 run to take a 21-11 advantage with 1:32 left in the stanza.
To make matters worse, Bell-Jeff standout guard Rishonda Napier picked up her second personal foul with 31.3 seconds remaining. Caila Hailey of Serra then made a jump shot at the buzzer to make it 25-12.
The Cavaliers, who finished second in the Del Rey League and reached the semifinals with a quarterfinal victory against second-seeded Oaks Christian, extended their lead to 34-13 with 5:36 remaining in the second quarter and ended the half on a three-pointer at the buzzer to make it 44-22.
Bell-Jeff played better in the third quarter, cutting the deficit to 46-30 on a three-pointer by Napier with 5:01 remaining. Napier, a reigning All-Area first-team pick, finished with a game-high 26 points. The senior made six three-pointers.
"She is a flat-out good player," said Hadley, who received 14 points apiece from Hailey and Nautica Marrow.
Napier made another three-pointer to bring the Guards to within 61-47 with 5:29 remaining in the game. However, the Guards could get no closer.
Johnson said the Guards, who got 16 points from Jasmine Smith, had a successful season and will see if they will qualify for the state playoffs.
"We overcame a lot of hurdles," Johnson said. "To get this far in this division is quite an accomplishment."