The landmine is a versatile tool because not only can you target the delts, you can also do so in a way that includes more upward rotation of the shoulder blades – much needed by most lifters and athletes. (Don't have a specialized landmine device? Just shove a barbell into a corner against a towel.)
1-Arm Landmine Press
Start in a half-kneeling position to better control pelvic and ribcage position. As you press up, try to "reach" at the top.
As you progress, you can attempt a tall-kneeling landmine press with reach and then a standing landmine press with reach.
Tall-Kneeling Landmine Press
Standing Landmine Press
The "reach" is crucial. Not only are you getting some great upper trap activity, but more importantly, a ton of serratus activity.