I'm aware that many people don't think Billie & Christy should have been able to take on the Charmed Ones. I am also aware that they give us an explanation in the comics but I going to leave that out for this.

Just to start off, I want to say Billie & Christy was not a match to the Halliwells. The prophecy says "...the most powerful witches of all time." so obviously they outmatch Billie & Christy. I also want to point out that the prophecy isn't saying they are these "all powerful" beings. They simply outrank any other witches in power level, potential, and/or potency as a collective but not necessarily as individuals. We even see the Jenkins need to summon the Hollow to have a chance to win and the need the magic of the Halliwell family simply to summon it.

I think the Ultimate Power was a threat to the Halliwells because of Billie's projection power, the same power Wyatt has. The power literally let you manipulate reality. I've always taken the power as a way to use magic without needing to cast spells or brew potion. It's a pretty strong ability and we've seen witch powers in the past that could have easily led to the deaths of the Charmed Ones. The deflection power could render the Halliwell's powers useless and Tuatha nearly took Prue Piper & Phoebe in less than five minutes.

I honestly think people are too hard on Billie & Christy while over looking precedents that show the Halliwells can be defeated by other powerful witches. Magic is more than power, its about wisdom too. A strong witch with little knowledge of magic could easily die. Just look at Bacarra. He only used spells & potions to eventually kill Paige & Phoebe. Christy was abducted by the Triad and spent 14 years learning magic, training with her powers, and studying the Charmed Ones in order to take them down meaning in the knowledge department Christy outclasses the girls who only studied for 8 years, 5 for Paige. However, I will give the Halliwells the fact they have more experience actually putting their knowledge to use.

All in all, the show may not have portrayed these points in the best way but with past evidence of the Halliwells fight, Christy's training, Billie's projection, natural innovative skills, and time with the Charmed Ones, I find it believable that The Jenkins coukd stand a chance against the Halliwells.