Yeah, like CloudyMusic said, you want the "+" number up to 10,000 as fast as possible. The number starts increasing the moment you hit 1000 on the big counter, but it builds much faster when you enable the A+B mode where the counter counts down, so you want to do that whenever you have a long stretch ahead with a lot of enemies to milk.
There are a few ways to do this, already covered in the guides available around here.
The biggest pointers I think are:
1. Shoot enemies with the correct shot for the biggest medal types. I think some enemies change the ideal shot when you are in this mode.
2. Let as many big medals hit the ground as possible, until they split into skulls. Bigger medals result in more skulls eventually. This might sound counter-intuitive, but the "+" number increases based on the number of items you pick up, not their type/value. During normal gameplay you don't want them to hit the ground, and want the big medals.

3. The big counter that counts down, will do so much faster while a shot button is held (and even faster when both are held), so try to not shoot except from when you need it. For example, take out popcorn enemies in small tap-bursts, etc. For the larger enemies that typically reward you for holding shot, you want to make sure you only do this while you actually damage them (or even better, only while they die).
4. Most enemies that take multiple shots will release a constant stream of skulls while your familiar is damaging them. So especially for the huge ones that are able to stay alive for a while, you might want to avoid damaging them with your main shot, while the familiar does the work. For example with Rosa, hold down shot shortly to fire off the red obs you can direct with the joystick, and then let go, and reap the rewards while not even shooting the enemy!