Ring of Pain Strategy for Hard Difficulty

Do not try to win during your first runs (normal mode). Anyway you will likely not succeed!
Take time to unlock equipments through achievements, all unknown paths and learn the monsters mechanics.
The guides from orian34 are very good and helpful.During you first runs in normal difficulty you need to focus on the achievements which unlock the good equipments.
Here are some of the good ones to unlock. I have unlock 56/84 achievements.

Winning in normal mode on the light and dark paths grant you the two initial items unlocking the hard mode.

I’ve found that playing with the double candle is easier than the dark stone because this last has a penalty in speed which is you main stat to reach the very important ability to first strike (and one-shot). Moreover, a high % of stealth is not a so good strategy as defense is scaling much better to help you pass the monsters. Playing with the double candle offers you better stats and is the easier way.

For the final choice between light and dark, choosing light seems also an easier way as you have only one last dungeon (room) to fight the owl boss in all its phases whereas some tricks from the dark bosses can be quite hard to overcome. I found very difficult to win against “Connection” in hard mode whereas the “chicken” fight becomes very easy if you have the “final form” spell lasting for the full boss fight.

What seems to work best is to build up for quite balance stats.
To optimise your stats for first strike and enough sustain with chance of blocking, the good balanced values to target at initial, intermediate levels and end game are listed in the table. Mainly remember the initial stats for a good start and end-game stats to target, as intermediate level stats are mainly an indication for the important speed threshold and reasonable corresponding other stats to be efficient at that moment.

Speed is the most important to reach some threshold. After 19 it is better to level up the other stats. For defense, it becomes also less efficient above 20. The more the better for attack.
Attack is what you need to perform one-shot on typical monsters from that level.
Defense is your main sustain to explore the dungeon until you can heal.
Your health should be more than 20.
Clarity is the less important but can help to increase healing. Clarity at 3 or higher is good (+2 healing).

You need an extra source of damage like poison, light or soul damage to beat monsters with high defense and clear the room. Poison from spell books is better than from equipment.

Random damage (or splash) is often provided with the secondary type of damage and is a good help to clear the room. And if you have healing on attack or kill, you will maintain you life quite well. Intimidation contributes to your sustain.

The secondary source of damage will help you to kill some monsters that have very high stats (health, or defense or speed with high attack) like Phase fiend. If you don’t have the way to kill them, ignore them somehow if you can.

Once you are correctly equiped with good abilities, after reaching the last level of stats (see table), you will start to snowball in the very last levels, gaining a lot of souls and more stat boosts.

Passing level 3, you need 30 souls to have a chance to buy an interesting item presented in the shop.
Passing level 6, you need to keep 60 souls for the shop.
You can do extra rooms at exit of level 3 and 6 before the shop selection, to farm more souls.Gold chests are rare but do not hesitate to open one because most gold items are very good and will really buff your chances. If you have enough souls, purple epic chests are also generally worth it.

Open rooms on alternate path according to what you need and what you are prepared for at that moment. Now you should be familiar with what you can find and the difficulty of each room.

Try to fill equipment slots rather than slightly upgrading one unless it provides a significant ability. At high level do not waste souls to reroll common or rare items in chests as you should target epic and legendary items.

The stats boost “one for all” (Specialised) can help to rebalance a weak stat but don’t take it if you don’t need as it can imbalance your stats and does not provide a net gain at low levels. The other stat boosts “Basic” and “Compromise” are better. In particular, take care about the speed threshold : now you can see which stats are affected.
You can risk the cursed stat boost or kill remaining monsters if you have healing means in this dungeon (potions or others) or the next (restful room).

Even with this guide, it remains difficult and you will need a little of luck sometimes to pass difficult situations. Failures often come from exploders where you can’t escape (trapped), or taking too much damage to flee them (or lasting poison). Some monsters will also hit hard for your balanced build and your sustain will not be enough (like Skulker) and because you don’t have the right equipment/spell at that time.

Here are some of my successful runs.