I have obtained hidden achievements in a vanilla version (I reinstalled SF after messing one file...), but I believe that it is possible to get them also while playing with a Community Patch. Personally I recommend playing DotP with this patch as it fixes a few bugs.

Note: It is impossible to get hidden ones without changing files because a proper code line is not implemented. Unless I am wrong, the same applies to achievement "Thou shalt not pass". I have no slightest idea what the original idea was for these. I just show how to implement them in whatever form.

(1) Open game folder - "SpellForce 2 - Demons of the Past"
Go to: \SpellForce 2 - Demons of the Past\addon3\map\Campaign\P701_Westguard\script
(2) Open file: "cs_02_GatesAreStillOpen.lua"
(3) Find these lines:


(4) Now add lines
in this place:


(5) You can additionally add the folllowing line in the same place in order to get "Thou shalt not pass".
(6) Start a new game. You will get "Kubi Rule" and "Urban Law" after beating the first groups of spooky scary skeletons, therefore it will take just a moment to check whether it worked out or not.

There are eight achievements which need not only grinding but also developing your avatar into different specializations. These are:

• True Dragon - Cast 200 shaikan spells
  
• Fire walk with me - Cast 40 Fireballs with the avatar
  
• Handsome - Charm 50 enemies with the avatar
  
• Fight fire with fire - Conjure 66 demons with the avatar
  
• Can't touch this - Get invulnerable 75 times with the avatar
  
• Cry baby - Cast Battle Cry 50 times with the avatar
  
• If it bleeds, we can kill him - Hit enemies with Slash 50 times
  
• Seven sins - Cast Salvo (Master) 40 times

Exact mechanic of these is described in another guide (I am not the author). I will only suggest a way of obtaining skills required for these achievements. It is possible to get above achievements without modifications through replaying the game and developing your avatar differently but it would take a lot of time even if playing on easy.

Please, use below modifications only for obtaining achievements. They grant your avatar a lot of experience points so natural progression will be disturbed.
Changes will be applied in map Remains of Steelcoast because of merchant who sells there weapons which enable usage of skills Slash and Salvo and because at this point your avatar surely can gain a few more levels.

(1) Open game folder - "SpellForce 2 - Demons of the Past"
Go to: \SpellForce 2 - Demons of the Past\addon3\map\Campaign\P707_Steel_Shore\script
(2) Open file "Rewards.lua"
(3) Find these lines:
(4) Now change it to:
Note: Do not change the gold reward, it crashes the game. Anyway, you will find infinite source of gold in Icewastes of Shalibar.
(5) Load a save before entering Remains of Steelcoast for the first time. Choose one where your avatar will have enough undistributed skill points so as to get to every skill required for achievements.
(6) At Remains of Steelcoast go to lava pool in the middle of the map. After a cutscene you will get enough experience points for 30 level. Make a save before distributing skill points. Now you can start grinding, Citadel is a good place for this as it spawns enemies infinitely.

Tips for grinding:

• Check merchants and your inventory. You might find accessories reducing skill cooldown.
  
• True Dragon might be obtained with Shaikan skills which you gain as you progress in story (Thunder of the Shaikan, Blood of the Shaikan, Time of the Shaikan) so it is possible to not spend a single skill point in a Shaikan skill tree.
  
• If it is possible for your character, pick at the same time these skills which have long cooldown and cannot be exploited (how, check the other guide of not my autorship) - Invulnerability and Battle cry.

A few achievements demand doing something on hard or very hard difficulty. Luckily, in DotP it is checked when the required activity ends, therefore you can do everything before on easy and then change difficulty to very hard just for completing a task.

Achievement "Relics of the Gods" requires finding 10 pieces of equipment set Guardian Elements. Their position in a following list represents the order of appearance.

• Niethalf's Bonecrusher - Westguard - On the eastern hill. Find a stone circle, head north afterwards.
  
• Elen's Singing Bowstring - The Stepping Stones - In the bushes on the Elen's island (second island). Change camera angle into the third person perspective, then search for a little chest in the bushes to the southeast of the statue.
  
• Tiara's Rising Sun - Realm of the Gods - Chest in a triangle room next to the southern gate. The room is located to the west of professor Twiddle's crystal.
  
• Ereon's Brainstorm - The Citadel - Behind the portal to the Shaper's library. Go north past the Archon Dor'Mat's encampment.
  
• Hirin's Harbinger - The Depths of Kul'Rath - In the gorge to the west of Resikk'Mass. Mount Jorbal and fly northeast past a waterfall nearby the Nameless base. Expect resistance.
  
• Shanna's Painkiller - Remains of Steelcoast - At the top of a pillar in the north. It is located close to a pentagram in the Malacay's laboratory.
  
• Zerbo's Spray and Pray - Shadow Realm - To the east of the southern journey stone (City of the Shadows).
  
• Nor's Moonshadow - Icewastes of Shalibar - In the southeastern corner of the map. The chest stands on the lowest surface.
  
• Kerona's Web of Time - Plains of Argan - Sold by Shaikan Merchant standing in the Shaikan base.
  
• Zarach's Bloodcleaver - Plains of Argan - Reward from the main quest. You cannot miss it.