Below are all the methods of training available in the game and their gain ratings in different areas. If a rating is not included then that means it does not increase the stat that isnt included.
Sparring: This might be the best form of training depending on how much effort you put in. It is divided into Active Sparring and Inactive Sparring. Active Sparring is when you are moving around and stuff, like a real fight. You can use melee or ki whichever. Inactive Sparring is when you are just standing still hitting something and don't need to put much effort in or be active.
Active Sparring:
BP: 10
Ki: 8
Stats: 4
SP: 12
Inactive Sparring:
BP: 3.34
Ki: 2.67
Stats: 1.34
SP: 4
Train verb:
BP: 2
Ki: 1
Stats: 1
SP: 1
Meditate verb: You need meditate lv2 to use meditation as a form of training. Otherwise it is just for resting.
BP: 0.4 times meditation rate (A Puran has a meditation rate of 4 meaning their meditation bp rating is 1.6)
Ki: 1
Stats: 0.85
SP: 1
Punching bags: Out of the core stats (strength, durability, force, resistance, offense, defense, speed) punching bags primarily train strength, speed, and offense, but also the others at a slower rate.
BP: 2.2
Ki: 0.5
Stats: 2
Shadow sparring: This form of training gives no BP. While the other gains listed are gains per second, this is gains PER HIT. So the faster you shadow spar the more you can gain. If you miss then your next hit will not give you any gains, this is how the game prevents people from just randomly hitting the buttons to gain.
BP: 1.5
Ki: 8
Stats: 4
SP: 6
Flying:
BP: 1
Ki: 4
Stats: 0.3
Gravity: See gravity section for more info. The more gravity you master the faster your BP gains will be.
Weights: By wearing these your BP gains will at least double while doing any form of training. Weights can be made thru science or the materialize ability.
Leeching: When you spar with someone stronger than you, you will "leech" their power gradually. Like a master training a student. If you are very weak compared to everyone, you should find someone to leech instead of self training, because your self training gains will be terrible if your Battle Power is low.
There is really no point in solo training if you do not have good weights and a good amount of gravity mastered, because those increase gains greatly, and the people who are using them will be pulling ahead of you like 20x faster, so you are actually falling behind, not becoming stronger, if you compare yourself to everyone else. So if you can't get good weights and gravity, you should just stick to leeching other people's power.