In martial arts all looks fine when the opponent is similar size and fighting "normal" way. I just wonder how to effectively fight against someone without much technique but much stronger (difference +20 kg) furiously attacking. Would you rather try to keep the distance and wait until he reaches the moment when the muscle s will need the oxygen or try to go for legs and beat him to the ground or kick low/middle kicks or teep kick or yet another option with hands like flying elbow?
The answer to this question really depends on:
What training you have
Where you are fighting (what terrain, obstacles, etc. is around you)
Every form of self defense has different options they specialize in. Jujitsu and Judo folks will get around the person's back and lock them up. Some folks will catch a thumb or get a wrist lock. Boxers will play with distance, jabs until the person opens up and then dig in. Untrained folks inevitably open themselves up a lot... and so on.
The only common factor in your question is untrained folks - untrained folks tend to throw telegraphed haymakers, tackle/knock down folks (followed by either a kick/stomp or mount and punch), and often don't have great balance themselves. That's about it. So it's absolutely style and practitioner dependent about how you respond to that.
Usually the biggest threat is the knockdown, because that's where untrained folks do the most damage. So that's either people playing good with evasive footwork to avoid that or good grappling to make sure they either stay on top (sprawl out) or take the person down hard (twist downs, etc.) and get good position from there.
My Answer Today
Given my judo and lifting training right now, I feel comfortable defending against throws by an untrained opponent. I feel less confident trading strikes. Therefore, I would stay outside striking range, maintaining distance using footwork plus an occasional leg kick, front kick, jab, or push kick as appropriate.
If the attacker persists I would find the right time to cover up, close the distance, and clinch. The goal is to transition from too-far-away-to-hit to too-close-to-hit-effectively. The power-striking range is to be avoided, since even an untrained opponent can land a lucky knockout blow. From the clinch, I would control strikes and posture, then go for one of the high-percentage takedowns or throws in my repertoire, such as kouchigari, osotogari, or another footsweep, or potentially a takedown from the back like tani otoshi.
Once I've taken the opponent down, I would maintain distance from a dominant position on top such as knee-on-belly, scarf hold, or mount. Again, this puts me in a position where strikes are just not effective for the person on the bottom. From there, I would finish the fight with words, a choke, a joint lock, or strikes (in descending order of preference).
What kind of training enables a gameplan such as above? Simply a few years of a throwing art like judo, sambo, or wrestling, plus a passing familiarity with sparring with strikes. Groundwork-specific training like BJJ helps with confidence once the fight has hit the ground. Basic strength training (barbell squats and deadlifts, pull-ups) means that I don't necessarily feel much weaker (or weaker at all) than an untrained person bigger than me by 20 kilograms.
My Answer Supposing Optimal Training
What if I had all the training I wanted? What would I do then?
Well, I would have better boxing, so I would feel more comfortable staying at range and throwing (probably open-palm) jabs and leg kicks to make this person reconsider their aggression in the light of getting bloody and bruised. Only if something went wrong would I clinch. Once clinched, I would rely on having better judo and wrestling than I actually do, enabling me to either disengage or take them down with a more devastating and decisive throw, like a hip toss or suplex. Other than that, the gameplan would look the same.