If my rets are sporting heavy flamers, I've been using Immolators and using terrain to make it difficult to pop them on the first turn.
Turn 1 to position and force the opponent to drop fire on the T7 chassis (which is much higher than any sister on foot), THEN if they have anything left, try to kill the bodies that spill out. If they cannot remove the Immolator (due to lack of firepower, good saves, or forcing saves with miracle dice when available), turn 2 they pop out and do their work. Typically, they are immediately shot down afterwards, or they snowball the table and take over that space.
Multi-melta retributors... I normally try to deploy them into cover for 2+ and try to stay stuck in it, threatening anything that comes into their range instead of trying to force them up the table. In a pinch you can spend the CP
to get the 36" range, but normally, 24" is more than adequate to threaten multiple objectives and make your opponent wary of giving you easy targets. But the cover feels like the most important part there, as the squad (even 10 strong) seems to just melt on that T3/3+ save profile if your opponent REALLY wants them gone.
In a lot of our crusade, I've been playing 5 rets with heavy flamers in an Immolator just looking for something, anything, to inflict massive casualties to... or to just draw attention away while I move around a bit more freely with other units. I've become fond of 5 Battle Sisters in a Twin Multi-melta Immolator with a Storm bolter and a Multi-melta. But basically, the idea is to make my opponent have to hit T7 as much as possible because T3/3+ won't cut it when you NEED it!
But the short of it is... the rets are going to get wrecked. If your opponent is prioritizing them, then you're probably using them right. If you want to protect them... castle them up with some cover (I've used the Battle Sanctum to ensure I get cover where I want it) and toss a Hospitaller down with them. This typically works best with Valorous Heart where you can ignore a bit of AP
and take full advantage of that 2+ (and possible 4++). Otherwise, wrap them up in some sort of chassis and get your opponent shooting at it instead of the gals on foot. The idea is to force him into that unit as much as possible before they're gone. At least from my point of view