Minecraft wiki entry for Half_hearted_hardcore, associated text: First of all, as stated before, you cannot take damage. So remember to avoid every single way possible of taking damage, albeit how pathetic, such as being stung by a pufferfish or being spat on by a llama.
Get as much food as soon as you can. Kill every animal mob you come across on your first day, apart from sheep which you should shear. Try to save 2 of each animal and make an animal farm as soon as you can.
As soon as you get enough iron, make a shield and keep it in your off-hand slot. It blocks damage from most mobs, including creepers and blazes.
When you get a water bucket, keep it in your hotbar in your inventory and never remove it, as it will save your life. (Make sure you learn how to use it.) If you get a powder snow bucket, replace that with the former, since you can MLG with it in the Nether as well as the other two dimensions.
When you get a golden apple, keep that in your hotbar as well and never remove it either, as, again, the absorption hearts will save you one day. If you are lucky enough to find an Enchanted Golden Apple, keep that in your hotbar as well.
If you manage to find a woodland mansion, wear at least full max-durability iron armour, a shield and bring a diamond sword, and then raid it. Pillar up and kill vindicators to prevent their axes disabling your shield. Evoker fangs can be counteracted by swimming in the top of water 2 blocks high, and remember that the fangs will most of the time only appear where you were when the evoker was starting the spell. Make sure you kill evokers before they summon vexes, but if they do, run out of the evoker's hostile range and block the vexes with shields until they start to die. Once you get a totem of undying, move the shield to your hotbar and keep the totem in your offhand, because it will save your life. You can keep switching between the shield and totem, but be very careful. If you can, try to use the shield in your main hand and attack enemies staying crouched with the shield.
If you manage to get a beacon, put it somewhere near your base of operations and make it Resistance 2 unless you already have one which gives you that effect. Resistance 2 will make any hits on you do less damage.
If you are in the End, bring a water bucket, even if you have powder snow, because it will stop endermen from getting close to you, it will damage shulkers, and it will help you get up pillars and end cities.
If you afk, make sure you use an AFK pool that is compatible for boats so you will not lose hunger. Enclose the box fully and make sure there are solid blocks over your head to prevent phantoms from spawning and lightning strikes from hitting you.
DO NOT use an ender pearl unless you are in a life threatening situation (e.g. falling into lava) and make sure you have at least enchanted diamond armour to reduce the damage to a non-fatal amount.