This idea was provided by Binary10101ERROR.

Note:Binary10101ERROR refers to shadowrath as ninjas.

If you have mid strongly secured but do not have enough damage to fight your opponent under their defences (walls, castle archers, etc) then camp at mid, unlock ghost spearton and buy one or two merics. Then research level 2 shinobi and buy Shadowrath. Everytime a ghost spawns, take control of your ninjas and follow the ghost, (staying behind it at all times)

Your targets then depend on the opponents defenses:

Giant heavy - Poison their giants - Even if they have healers (without cure) this will keep pace with heals

Merics with cure - Tier 2 shinobi one-shots merics, assasinate them. - Opponent loses 300 gold and 200 mana every wave

Walls - Tier 2 shinobi takes away almost all of a wall's hp. Your opponent will not lose units, but wasting 200 gold on a wall every time a ghost spawns will reduce their economy to nothing. 

Eclipsor/Albowtross heavy- Rush past the albowtross and attack squishies behind them 

Magikill - Strafe to avoid spells, back off if you can not easily get them. Poisoned ninjas should head back to mid immediately.

Magikill behind walls - Focus the wall first and remember to strafe

Archer heavy - Archers are expensive but can be spammed easily late game. Attack them only if there are no other more expensive targets or if they are small in numbers, losing one or two archers means little when they can be re purchased in 15 seconds.

Juggerknight/Spearton heavy - People buying speartons excessivly may be planning to rush past your army when it attacks fully, you could opt for stalling untill your opponent runs out of gold, but assasinating when your opponent has 12 speartons is difficult. You may be better off to wear down their statue. Otherwise, having 3 magikills proves extremely efficient since they group up and are vulnerable to AOE attacks. Having a few castle archers help as well, but all above all, remember to upgrade your statue health.

Speartons and juggerknight massing can be prevented with constant flying archer harrasing, either shoot them from air permanently, retreat when your flying archers are weak from the castle archers, or sacrifice them if you have enough time and mana and gold. Ghost giants help a lot too.

Ninja heavy - They shouldn't be defending with ninjas, but try to time shinobi to avoid theirs. 

Dead heavy - Dead castle archers - Be very cautious to avoid poison but do target deads as the 100 mana cost makes them difficult to re-purchase repetedly.

Note: Miners are usually in excess by the point you can perform this strategy so it is best to attack expensive units rather than miners, as this will cause more damage to your opponents net army gain. 

After one shotting or harming a target you should back out with your ninjas, in most cases the ghost will still be covering for you though against albowtross with blazing bolts you may sustain heavy damage. Heal up your ninjas at mid and wait for the next ghost spearton.

This strategy will heavily wear down your opponents army, though it is best to fully attack if your population count maxes out as if you wait any longer your opponent will gain more army strength but you will not.

Note: Performing this strategy without a strongly secured mid may mean that your opponent chases your retreating ninjas and takes mid from you when they would have otherwise continued defending