No1 is giving you antilead. You have to buy it.
There are a few anticheats released. But idk if they still work.
Hook the .dll file in your runnable. (theduel.exe or gunz.exe).
If you don't know how:
[Tutorial] How To Hook DLL For newbies
If you are lazy or dont understand the tut:
[Service] Hooking DLL
And if you want to do it yourself (try without hooking) you need ASM. Not C#/C++.
CALL DWORD PTR DS:[LoadLibraryA]
CMP EAX, 0
CALL DWORD PTR DS:[MessageBoxA]
CALL DWORD PTR DS:[GetCurrentProcess]
CALL DWORD PTR DS:[TerminateProcess]