2 hands and ideally you don't have a device with a defective touch screen or by now you know your device well enough that it requires less more pressure. Of course all these methods become trickier when you're doing multiple corner/location drops than I feel like there's just more tapping involved. Usually for me, 2 hands and first dropping the generals and mercs in whatever area they go than I select my next unit (Usually apcs) and than left hand 3 finger tap the screen to drop them and once they're done than I quickly switch to next unit by selecting with right index. It's weird but it's the only consistent way I've been able to get it to work. The trick is timing of being able to select the next unit immediately because if you do it incorrectly than you have to pick up the left hand and tap screen again and you lose some time. I suggest practicing on wherever it doesn't cost anything like challenges or single player.