That's just the first fight. You fight Izaro a total of three times each time you Ascend. Twelve times in total if you go all the way to uber lab.
Anyway, I usually put off normal lab until cruel, cruel lab until merciless, and merciless until some time into maps (somewhere around level 80-85). I would probably do it that way even on softcore to not waste time. Uber is partly decided by RNG, so if my build is capable by the time I unlock it, I do it then.
There's a bunch of mechanics during the fight which can buff Izaro. They carry over into the next round. The most dangerous are probably curses (kill totems), charges (click statue just before you kill him, timing that sucks) or extra ele damage (golems that slowly walk towards him).
Bring a stibnite flask a granite/basalt and, my favourite, a decoy totem next time you do it. The flask will protect you reasonably well from subsequent hits and he all but magnetizes to the decoy, which is very, very useful to have more control over the fight.
If your damage is amazing because of twink gear, disregard everything and just burst him down.