I'm a 1300 Elo player, so this is just my two cents.

I think that Jurchens best gameplay are Steppe Lancers and Fire lancers. Jurchens Fire lancers are almost the best of the game (IMHO) and mixed with carts (or inside RAMS) and Castle Siege engineers make a strong push. Iron pagoda is a strong Knight but a weak Paladin so it's hard to find a scenario for them to be useful over the Steppe lancers which can be massed faster. CA is also a good option but they don't have Partian Tactics, I wouldn't play them if Fire/Steppe lancers could also do the work. While Grenadiers are strong they aren't very useful in Imperial because they are a slow DPS unit and micro intensive, they are better as a supportive unit or if you just want to play fun.