Actually even if he's fast, he's not that hard. When he goes on his pounce stance, look at the direction his tail is pointing at. That's where he'll go. And watch out for the tail slam, it's a OHKO move. He's also weak to flash granades and susceptible to pitfall traps when he's enraged.
G-rank gives Narga Brainstems a lot more, so i suggest farm that instead. And you also get a chance to get Narga Heavenly Scales when farming for brainstems.
You I honestly have no idea what the update did since it gave us no information -_- However we did link really fast. Though it'd take more tests to find out if that's what they fixed.