Stay above 2500 if you can, but either side of that mark is plenty of weak TH10 and lots of TH9 to farm... above that the TH9 dry up and the TH10 are stronger. My own TH9 seems to hover around 2450, and finds lots of easy hits for 20 balloons the rest minions; I carry 1 lightning 1 heal 2 rage but try not to use them all. It gets hit by hogs or giants + hogs a lot, so a solid anti-hog setup is definitely recommended. Or chuck your TH out - loads of snipers will gladly take it on their way to Masters. Above 2600, the bases are definitely tougher on average, and IMO that's best for TH10 farming not TH9.
