Basically the best, cheapest way to train invention (through combat that is) is using Augmented Black Salamanders. You'll need about 2 of these to level 5 to achieve level 27. At this point you'll be able to research Augmented level 10, do so. At this point it might be nice to try and get a few perks. Do not try hard to achieve the best, as this will be costly and later on the better perks are easier to achieve as invention level increases. This is basically also to road you'll take towards 99/120. Research Junk chance reductions and Drain charge reductions asap. Try to predict/calculate the amount of Augmented weaponry you need to reach the levels where you can research the drain/junk reductions. Alternatively, you can go down the skilling road. It is very cheap, albeit a bit slower. The best way to do this is Rod-O-Matics, which can be researched and used from level 22 and onward. Same principles applies, level it to level 10 and disassemble, etc.