Half guard top is a very fruitful place to look for submissions. The conventional wisdom, to pass the guard first, is good advice for when you first get started operating from inside the half guard, but after a few years, it can be prudent to start attacking much sooner in the process, when your partner least expects a submission set up. Here is a combination that expands on the classic Kimura-Guillotine combo, along with some details on different ways to catch the submissions early on.
"When attacking the Kimura, the best way to get a faster submission is to minimize the effective range of motion. One way of accomplishing this is to bend the arm at a 90 degree angle. By forcing the shoulder up as if your opponent is shrugging you make the range of motion even smaller. A natural defensive reaction is to separate the shoulder from the neck, but this in turn opens up the option to attack the neck," Cody Maltais writes.
Cody says, "Reverse half guard passing is a great way to neutralize an opponent with a strong underhook. Additionally, you will consistently get an opponent who tries to pass the arm under your arm pit while you are facing their legs and in this moment you can capture the Kimura grip. A key point of focus is to add a lot of pressure on their hand pushing in to their hip."
"Once you have the Kimura grip set up adding a little pressure will get them to push their shoulder away from their neck. Maintain the grip on your opponent's wrist throughout as you transition to a mounted guillotine. Notice on the finish rather than lifting the head, the goal is to crunch to the side and keep the grip secure and the weight heavy."