Damn it's like searching the whole world for a piece of answer.

In relation to daimonin_object.c line 1300 in src/plugin_lua or this topic

https://www.daimonin.org/11609/core-lua-methodobjectsetskill/msg103871/#msg103871

there is a limitation in this function. First you can only go 1 level up or down, but that's not the problem. Script has a build in protection for indirect skill.

line 1472 daimonin_object.c
/* If ->item_level == ->level, this means it has already gained
 * some experience via a script this level so the player will
 * have to go back to normal grinding for experience until next
 * level. This prevents scripts being exploited too much to
 * gain mega-levels. Return 4, skill, 0, 0. */
 if (skill->item_level == skill->level &&
 (level > 0 || (level == 0 && exp > 0)))
 {
  failure = 4;
  level = exp = 0;
}

Why this? No normal player can start this script. Only a mapper or a scripter can do this. And only dev-team can implement maps or scripts on your servers. So with lua-script i currently didn't see a way to increase levels on indirect skill without this limitation?

I can also use GetSkill() to gain read access, but there is no other way than SetSkill() for write access.

local sobj = pl:GetSkill(game.TYPE_SKILL, skill_sling)

So i can read obj.experience, but can't write.

Writing obj.experience is unreachable from script level, except this one function? Currently i see no way to increase an indirect level only by lua script, without this strange blocking behavior.

Other way is to remove this block an recompile plugin_lua.