There are quite a few ways to do this... Toggle god mode, set health to be incredibly high, give 100% evasion (doesn't always prevent death...)...

If what you're looking for is unlimited health, though, I'd probably have the Ultimate skill set a key value to 1 (and then delay set it to 0 for whenever it ends), and set the player's health to be extremely high. Then, I'd create an event player_hurt code so that whenever a player with a true value for that key value is hurt, their health becomes that extremely high number again. And finally, when the effect ends, set the player's health to something more normal (50, 100, whatever it was at before the ultimate was used, etc).

Just set the players health above 500 at roundstart, and everytime he gets hit, add the amount of health he got hit, or just add a fix number like 500.
Simple as that.

I think it'd be something like this:

GeSHi (eventscripts):
    "skill5_setting"    "es_xset wcs_duration 4"
    "racealias_inv"     "es_tell server_var(wcs_userid) #multi #lightgreenYou have become invincible for#green server_var(wcs_duration) #lightgreenseconds, and will be reduced to#green 50 #lightgreenhealth when this effect ends"
    "skill5_cmd"        "es_keysetvalue WCSuserdata server_var(wcs_userid) invuln 1;es est_sethealth server_var(wcs_userid) 10000;es_delayed server_var(wcs_duration) es_keysetvalue WCSuserdata server_var(wcs_userid) invuln 0;es es_xdelayed server_var(wcs_duration) est_sethealth server_var(wcs_userid) 50;racealias_inv"
    "skill5_sfx"        "0"
    "ultimate_cooldown" "45"
 
event player_hurt
{
es_keygetvalue wcs_tmp WCSuserdata event_var(userid) invuln
ifx true(wcs_tmp) do
  {
  es_xset wcs_dmg event_var(dmg_health)
  es est_sethealth event_var(userid) 10000
  }
}

You can just make a skill that when the ultimate is used you set the user health to a value, and after some seconds it setted back to a value that you want:

Or you can set a keyvalue getting value from the user health, the set the health to the value you want, after some seconds you get value and set the health of the user for it... It requires a lil time to code...

I'm disappointed of current wcs races development and requests.

I don't recommend doing that whole thing after all.
And I have to agree with Stinkyfax's statement, as far as I could see since I'm back, it's...the questions are...whatever.

@volkano

FIRST LOOK FOR THE EST COMMANDS THEN ANSWER