Mechanics:
The skill can only be targeted at a range of 500 units on enemies or the ground, but it hits units up to 950 range away. This is because the Earth Spikes travel for a length of 825 units but hit in a radius of 125 (hence a range of 825 + 125 = 950).
The wave of spikes moves at 1600 units per second – this means that the units at the end of the Earth Spike (825 length) will be hit after 0.52 seconds. The closer you stay to a target, the sooner the Earth Spikes will stun it. In that sense melee range, Earth Spike are the easiest ones to hit reliably and almost instantly.
Earth Spike can hit and stun invisible units. You can actually tell if you have hit or missed by the damage animation on the spell. If the animation triggers, then you have successfully landed the blind Spikes. This information can be very useful to your allies that can cast some follow-up AoE spells and secure a kill.
Aiming:
If you are too far away from a target to cast the ability on it (the target is outside 500 range), you can target the ground in the direction of the enemy – the spikes will hit him even if he is 950 units away. This tactic is very useful for hitting escaping heroes – if you target such a hero directly and he moves beyond 500 range, your own hero will uselessly run after the target instead of casting the spell.
When targeting heroes very close to you it is a good idea to click on the actual hero – this way even if he tries to dodge, Lion will cast the Spikes in the right direction (the close range stun is almost instant). When there is some distance between you and your target, however, it is better to predict his movement and target the Spikes on the ground where the target will be when the projectile reaches him. If you click on the target, the mentioned long range delay of the stun might cause you to miss.
Strategic use:
Earth spike scales well with levels – both the stun duration and the damage get increased. Therefore it is worth maxing Earth Spike as fast as possible. The ability is both your AoE disable in fights and a farming/harassing tool.
In team engagements, you should try to get multiple targets disabled. To do this you must position yourself in such a way that you and your targets are standing in a straight line (of course, this is much easier with a Dagger). Afterward you need to target the ground on that “line” rather than a specific hero unless you value the disable on him more than the disable on the other target(s). This way even if one of the targets moves, your hero will still cast the spell in the right direction.
Even though Lion isn’t a hero with tremendous team fight presence if you learn to consistently land multiple-hero Earth Spikes you can be a great team-fight initiator (or counter-initiator).
In ganks, it is best to use the spell as a follow-up disable to your Hex. The reason is that the Hex is instant, it is much more reliable and it serves as a set-up for the Spikes (it is impossible to dodge the Earth Spike with 100 MS). If you don’t overlap the stuns you can disable a single target for more than 6 seconds. If for some reason the Hex is not available, you can use a mobility item to get in close range to the target and cast Spikes immediately afterward – melee Spikes are also almost impossible to dodge unless your target has some instant escape mechanism.
When it comes to farming, you can use the spell on CD because you have Mana Drain that can sustain your mana.