I'm currently rocking my Grand Company weapon (I've had terrible luck with primals) and my FC BLM has his Relic +1. It's not EASY holding threat with him constantly trying to steal it (a gentleman's game we play) but for the most part I perform admirably and we have smooth and fluent runs. In addition to this we sometimes run with a SMN who puts DoTs on every mob and so on. The best way to deal with it is to experiment with a rotation that works. However, the people you were running with were purposefully sabotaging your attempts and relying on their gear to carry them. Honestly they were probably laughing it up in vent or something just having fun messing with a random tank. I wouldn't worry too much about this incident. But with an Ifrit weapon you should be able to tank for Relic +1 DPS who are cooperating and attacking marked targets, letting you hit first, etc.