Muyo

It's pretty simple, first thing you'll need to know is that max for tick_def is 10000 (100%). Out of this and luk that you want immunity on you solve simple equation:
x = 10000 / ChosenLuk;
 
After that just change this line:
 
tick_def = st->luk*40; to
tick_def = st->luk*x; Where x is the result of equation.
Example: You want immunity on 300 luk, that means
x = 10000 / 300;
x = 34;
(It's for you to decide where to round, up or down. I rounded up. That measn that actual luk you'll need for immunity is 295. If you round it down, then needed luk will be 303)
So the resulting line will be:
tick_def = st->luk*34;

try add
 
tick_def = st->luk>249?0:tick_def;

Umm, tick_def is the reduction. In your case, you just set it up to 0 Of course it had no effect. Try it like this:
case SC_FREEZE: sc_def = st->mdef*100; sc_def2 = st->luk*10 + SCDEF_LVL_DIFF(bl, src, 99, 10); tick_def = (st->luk>249)?10000:0;

I gave changed line, this mean need remove old "tick_def = 0; //No duration reduction"

Wait wait wait. According to the code you gave there was no reduction to the freeze time whatsoever, just after 250 luk you were pretty much immune to the status change. Like with curse and 100 vit - status still strikes, but is removed same moment.
Can you clearify what do you want? So that luk would bring linear reduction to freeze time? Or % reduction? How much each point of luck should reduce?

case SC_FREEZE: sc_def = st->mdef*100; sc_def2 = st->luk*10 + SCDEF_LVL_DIFF(bl, src, 99, 10); tick_def = 0; //No duration reduction tick_def = st->luk>249?0:tick_def; to this.
case SC_FREEZE: sc_def = status->mdef*100; sc_def2 = status->luk*100/35 + status_get_lv(bl)*10 - status_get_lv(src)*10; tick_def2 = status->luk*10 + status_src->luk*-10; // Caster can increase final duration with luk  
or refer to this http://rathena.org/board/topic/99448-about-frozen-and-luk-resistane/

Sigh. The example above will give you at least 2 compiling errors.
 
case SC_FREEZE: sc_def = st->mdef*100; sc_def2 = st->luk*10 + SCDEF_LVL_DIFF(bl, src, 99, 10); tick_def = st->luk*40; Try this one, should reduce it in % manner. Every 25 luk will reduce duration by additinal 10%.
Like this:
25 luk -> 10% freeze time reduction
50 -> 20%
75 -> 30%
and so on. On 250 you'll reach immunity.