Warning: You need high pain tolerance to do this shoulder workout! This is similar to the good old 21 technique you've probably used for biceps, which is also based on constant tension and lactate build-up, but we'll do sets of 5-5-10 instead of 7-7-7.
Dumbbell Press Multi-ROM
A. 5 partial reps doing only the bottom half of the movement
B. 5 partial reps doing only the top half of the movement
C. 10 full reps
The partial reps will cause a rapid hypoxic state (lack of oxygen) and a rapid build-up of lactate, whereupon you do your full reps in that state.
On the full reps you want to do the reps smoothly, not stopping at the top or bottom between the phases of the lift. Stop about half an inch before locking out to keep the deltoids under tension.
Do 3 sets with about 90 seconds of rest between sets.