How practical is laser designation against helicopters?
There are a number of laser-guided ground-to-air systems, and a number of those have been adapted to air-to-air and air-to-ground as well. The key is the speed of the missile, not so much the guidance system itself. An example is Starstreak.

Lasers are well focused, but not perfectly, and are easy enough to defocus to any required degree. For this role, making the beam large enough to cover potential movement of the target during the tracking cycle (often manual) while still having enough energy fall on the target for the tracker to seek is pretty much all you need. Modern solid-state lasers make this relatively easy. However, unless you have a seeker in the munition, as is the case in a laser-guided bomb, then the cone-shaped beam means decreasing accuracy with range.

Starstreak takes another approach. It projects a 2D array of dots in space modulated so the receivers on the darts can tell where they are in the array. When used with the shoulder launcher, the operator keeps the beam on the target optically and the darts keep themselves in the center.

When combined with the optional launcher platform, a tracking camera is added. This receives the reflected laser signal from the target and uses the same modulation to determine how closely the array is to the center of the target. It uses this to automatically track the target, and thus the missiles get the benefit of semi-automatic tracking without having to carry the seeker on their nose.