Mordekaiser can make excellent use of his Iron Man shield at all stages of the game, from absorbing harassment to giving powerful survivability during team fights, letting him stay in the field longer than most other champions. However, keep in mind that spamming skills will result in consistently low health without sufficient health regeneration or spell vamp.
When Mordekaiser's health falls below the cost necessary to use a skill, his health will drain to 1 health. Any subsequent spell casts will bring him down to 1 health, meaning that he cannot kill himself through his own abilities. Keep this in mind if you are confronted when at extremely low health, as you may still be able to survive through absorbing damage with your Iron Man shield.
While you can block damage with your shield during laning, it is generally wiser to farm very conservatively during the early game instead, as Mordekaiser lacks any form of escape mechanism and pushing the lane in order to charge your shield increases the risk of an opposing gank.
Creeping Death is usually the best ability to gain shield health, due to its consistent AoE damage.
Mordekaiser's large array of damaging AoE abilities lends him to be a formidable teamfighter. His Iron Man shield is a powerful asset, letting him remain in combat for extended periods as long as he makes constant use of his abilities.
Children of the Grave should primarily be cast on strong autoattackers (particularly the opposing carry) in order to maximize your damage output and maintain your shield, but do not be afraid to use it on lesser champions in order to secure a kill.
Creeping Death does not necessarily have to be cast on yourself. It can often be more beneficial to cast it on an initiating tank, who will greatly enjoy the increased resistances and may often be in a better position to damage multiple opponents with the AoE, helping you build your shield more quickly.
Mordekaiser's abilities offer different quirks depending on which is prioritized for leveling.
Leveling up Siphon of Destruction gives you the ability to poke and is excellent for absorbing harassment due to building up his shield. Its health cost can become prohibitive at later ranks, however, leading to consistently low health if it is recklessly spammed.
Leveling up Mace of Spades gives him exceptional damage if he finds himself regularly exchanging autoattacks with his opponent, as it resets his autoattack timer. This is especially so if the target is isolated. It offers practically nothing against strong ranged poke, however.
Creeping Death is Mordekaiser's most damaging basic ability, giving him an advantage in prolonged duels, and the AoE lets him push and farm creep waves with extreme impunity. As it pushes the lane so strongly, however, it should be carefully used in lane to avoid inviting jungler ganks.
Like most other secondary units, the soul you've stolen with Children of the Grave can be controlled using Alt + Right Click. However, unlike most secondary units, your ghost acts as an allied champion, and will be affected by abilities that can only target champions, such as Wish, Requiem, and Infinite Duress.
If you do choose to use Ignite, keep in mind that its range is much shorter than Children of the Grave, and so you should try to use it first.
Champion ghost will attract tower aggro first, but if it attacks a champion while you are in range, you will become the new target. If you cast Creeping Death on the ghost and it hurts an enemy champion, you will draw aggro instead of the ghost. Be wary of this when attempting to tower dive weakened enemies.
The same virtues that let Mordekaiser teamfight effectively also lets him farm and push extremely quickly.
Mordekaiser is very capable of taking a solo lane against two enemy champions while maintaining his farming income, as his kit lets him easily freeze the lane at an comfortable point while simultaneously harassing his opponents. With a sufficient level advantage, he can even fight a 2v1 and obtain a double kill with his ultimate.
Mordekaiser's strong pushing ability can be an asset during laning by letting him roam effectively - he can quickly push the lane to his opponent's tower, and then leave to either assist another lane or farm the jungle for additional gold.
Although his kit is better utilized in a teamfight, Mordekaiser can also act as a reasonable split pusher due to his sheer wave clearing speed. As always, it should be ensured that you do not run the risk of becoming a victim of a gank.
Take advantage of the fact that many people underestimate Mordekaiser's damage once he gets his ultimate. When combined with Ignite and his other abilities, he can burst down and enslave enemies who get in range of him deceptively easily.
In choosing your target for Children of the Grave, consider if you'd rather kill off an AP based champ and use that champion's stats to make yourself stronger or if you'd rather kill the enemy AD carry in order to kill the enemies that way, in both cases you gain a substantial stat boost that can make a difference in a big fight.
Early purchases of items that can compensate for the health cost of Mordekaiser's abilities should be strongly considered.
A Hextech Revolver bought during laning can greatly increase Mordekaiser's sustainability and lets him harass more freely, in addition to increasing the damage of all of his spells and improving his shield generation.
Another option is Warmog's Armor, which also provides significant health regeneration via its passive.
Iron ManMordekaiser passive grants him an extreme amount of effective HP as a shield. Mordekaiser gains 35% of the damage he deals to champions as a shield. Furthermore the generation of this shield isn't diminished by AoE effects, which are most of Mordekaiser's abilies, nor by Grievous Wounds. Effectively this means he can generate his shield as fast as if he had 105% spell vamp. That being said, improving the effectiveness of his shield by increasing Mordekaiser's damage and resistances should label him a serious threat for anyone to face alone, or even in a 2v1.
Zhonya's Hourglass and Abyssal Scepter will vastly improve his damage and thus shield generation, while at the same time makes his shield extremely tough to break through.
As Mordekaiser lacks innate CC, items that can help prevent him from being kited easily are important. Rylai's Crystal Scepter, Hextech Gunblade and Twin Shadows all provide itemized CC for him and their own unique benefits. A Randuin's Omen can help prevent him from being kited by autoattackers by slowing them on hit and augments his survivability enormously.
Granting Mordekaiser extra mobility also greatly assists him in sticking with his opponent. Twin Shadows and Boots of Swiftness greatly boosts his speed, grants him an initiating slow, cooldown reduction, decent amount of AP, and resistance to enemy slows. All greatly helping him catch, stick to, and eliminate anyone caught out alone.
A lot of Mordekaiser's magical damage comes from the base damage on his spells, as his ability power ratios are fairly low. Consider prioritizing magic penetration and cooldown reduction over pure AP. This is doubly important on Mordekaiser as increased offensive prowess directly translates to improved survivability through his Iron Man shield.
The pet that Mordekaiser gets from Children of the Grave is considered a champion. It can not be affected by abilities that target only minions. That also means all abilities that target only champions will work on the pet as well.
Commit to taking Mordekaiser down. Iron Man can easily drain away, but will block quick pokes. Crowd control effects such as a silence or stun will also prevent him from growing his shield back for a short time.
Mordekaiser can become surprisingly tanky when surrounded by enemies, as his Creeping Death will constantly heal him and refresh his shield, while Mace of Spades and Siphon of Destruction will constantly allow him to deal nice damage. Children of the Grave can also grant him a constant amount of healing, especially when enemies with high Health or low Magic Resist are targeted. Champions like Ashe or Zyra can easily keep their distance from him but also continue to damage him in the process.