How far can a solo CoC mage go? (in General)

If I am willing to forfeit to archers and possibly SFBM and concentrated MM strats, would I be able to get enough damage to kill everything in 2 rounds while staying alive for AMF and GA retaliation? Also fight at 10-20% challenge bonus at the same time?

No, maybe, and most definitely not.

hypothetically, if I get 20m HP, 20m COC and 20m DM at 3m MPR, I would beat everything? (I am dream, can't I?)

I suppose if you could hypothetically get a +4835 leather cap, you'd automatically win too.

something to aim for PK, but if you got a familiar and junctioned you might do well.

maybe CoC with heavy AC?

You'd do better to just junction a coc familiar.

I've run a number of CoC mage teams. A single CoC mage/archer works quite nicely because you do damage in ranged, but to survive until the HoC provided first round of CoC used to require either big DBs or evasion, and these days that requires too much XP dilution to get the dex. AMF is easier to reduce than it used to be, but GA is a killer. SG would be a better option: less backlash from AMF, which means you could forgo NSCs and go for Alatars and get HUGE damage...

The legendary heavy AC mage could work, and would work better with SG than CoC because of the concentrated damage. CoC's spread fire come in too late IMO.