Am reallly struggling , it seems like I just get taken out in like one or two hits. Is regnar not good for Dom, or just a bad build?

He's not tanky enough, or not derpsy enough - one of the two. He's too easily kited outside of grass, and seems to die inside of a stun. If you want someone who does his job better, pick Warwick or Jax.

Im having issues with this myself. I really like the guy, but after trying phreaks build, cdr/triforce build, even mallet plus IE or SB and though all seemingly work on their own, each feels like their missing something from the other.

it is like he needs a jungle/minion farming period to build up ferocity before he can attack a champion

Right. That's kind of why I like to max cdr with him. So your ult can remain handy when you need the ferocity, or anything else it can provide really. And at a 30 second cd, you can get a lot of mileage out of it. My current cdr build leaves me kinda lacking in offense and really lacking in defense. I'd be interested in a good cdr defensive build for him

ive had some decent success.
revive/garrison (might replace garrison with something else)
21/9/0
arp reds
armor yellows
cdr/lvl blues
movespeed quints
R > Q > W > E

bonetooth
boots 1
2 hp pots
brut
boots 2 (merc/tabi)
maw/atmas (depending on if u need armor or MR
last whisper (if needed)
triforce

my issue with cdr is that it doesnt actually give u more dps. rengar deals dmg almost exclusively through his q combo. if u dont kill someone inside that initial combo/initiation, no amt of cdr is going to help you. think of it like veigar. if u dont kill someone in that initial burst of yours, ur not really useful in a teamfight anymore

as far as tankiness goes, its the same issue. no amt of it will get u closer to surviving a teamfight the same way as a tanky dps will. think of him moreso as an assassin than the fighter he's classed as. his dmg is bursty so u should build just enough defense to survive the initial burst of any champ. if u dont kill someone 2 or 3 seconds after your initial Q combo, its over. anyway.

I've been building the following in a 5 man team with friends and we're pretty competent with the first top engagement :

Boots, bonetooth, pots
Youmuu
Maw/Mallet depending on needs
If maw was picked, mallet. If mallet was picked, bloodthirster.

PS. His name is Rengar. :3