I love Hs129 but I stopped trying to use that 75mm cannon and switched to Mk101/103. Probably I needed more patience. Or just to give up and to wear glasses as my most precise shot was always the last one before the crash.
As for IS tanks... In older (4.07 and earlier) I flew Ju87G a lot. IS were real pain in the ass especially online (with network lags, enemy fighters, etc.). But high angle attack with shot in the rear usually worked.
By the way, Orel War server (185.91.117.32:21015) has one map which is excellent for Hs129 practice. This one:
http://www.letkaorel.eu/fusion7/foru...?thread_id=238
There are T-34s, IS-2 and SU-152 monsters - all moving. Mixed with mobile AAA and soft targets. Cool map.