I've analysed a replay and came to these preliminary conclusions.

TL;DR: I'd estimate 20% base damage/HP, with the other 80% coming from skulls.

• Defense: golems have equal base HP (about 20% of its max HP for 100 skulls), and the rest of golem HP comes from an "pool" of HP, divided linearly based on number of skulls.
• Attack: similar, with a small base attack number for each golem, and additional damage divided linearly based on number of skulls.

In addition, there seem to be "rounds": each subsequent "round" of golems will be stronger in absolute numbers (HP and damage wise).

Finally, the time mines have been opened also has some influence on golem health and damage, although that has recently been capped from the moment 4 minutes have passed or at least 60 skulls having been collected. See these patch notes for details.

Defense

The first three golemns of my match were like this:

• First Golem

  

  • Blue team: 25 skulls, giving about 20 health bar ticks
  • Red team: 75 skulls, giving about 37 health bar ticks

• Second Golem

  

  • Blue team 78 skulls, giving about 63 health bar ticks
  • Red team 22 skulls, giving about 33 health bar ticks

• Third Golem

  

  • Blue team 87 skulls, giving about 95 health bar ticks
  • Red team 13 skulls, giving about 33 health bar ticks

If we convert this the percentage of skulls and a percentage of total health bar ticks amongst the two golems, and plot that, we get this (with a linear trend line):

I have not been thorough enough for this to be acceptable on our statistics sister site, but based on experience and gut feeling I'd say this trend line is close to the real thing.

Offense

A similar thing can be done with offense. If you observe the health of the same building (e.g. a wall) drop for golems of different strength, a graph could also be plotted.

This is significantly more labor-intensive, so I've left that as an exercise for the reader. However, based on casual observations, I'd estimate a similar graph would pop up as the one for defense. To give an indication, here's a comparison of one single hit of golem 3 mentioned above on a wall:

This translates to:

• Left / 87 skulls: 13% damage on a wall per hit.
• Right / 13 skulls: 5% damage on a wall per hit.

If anyone wants to collect more / more precise data, I welcome them to add it to this answer. But perhaps we should wait until we can (/ know how to) use the combat log :-).

The biggest hurdle with games of this nature are balance heroes against each other. There isn't a clear guide with how each 1v1 should break down since some heroes are generally stronger than others. Similarly, there isn't a clean concept that one role always has an advantage over another.

A very loose concept could be something along these lines... loosely:

Specialist > Assassin > Support > Warrior > Specialist...

However, there are FAR too many cases where this logic doesn't stick. Diablo, as a warrior, probably can't do too much to a Gazlowe who lives on his turrets. Whereas Sonya probably has a good chance because of whirlwind healing... but only if the Gazlowe places turrets in such a way to support this.

Your largest improvements will take place once you get accustomed to what each hero excels at. They all have their perks they bring to the game and the role itself doesn't do anything but generalize~.

(Also, Diablo is generally considered low tier due to his skill/talent options).