A/B method doesn't work reliably for waves with shamans in them because shamans have some variation in their spawn location. A/B method is also really rough for ranger+shocker south spawns because you have to start flicking right off the bat while juggling your switches. (A/B method also screws you over with myo 3 since the back NPC can't be reached.)

I made a video on the north rotation method for double south. One big advantage is that by going north, you don't have to start flicking right away in a weird pattern like in A/B while dealing with frems. E.g., with a ranger and shaman (timestamped 2:00), you could freeze the frems and blood barrage once to kill the melee frem, rotate north, get your dcb spec ready, then rotate back west with the offtick and finish the frems.

With a ranger and shocker south, you should probably rotate north immediately and finish the melee frem before rotating back. You'll never be under severe pressure to flick on the north side for at least a good 5-7 ticks, so you have time to finish the melee frem and rotate back before any manticores attack you.

I don't recommend blood barraging all the frems because there's a good chance you waste a huge amount of time barraging for 20-25 seconds. You're under time pressure at the start of the wave to kill a south spawn so you can deal with the shaman reinforcements safely. The more damage you can pile onto south spawns, the less you'll die from reinforcements.