Question. Do you get any less experience for lower level monsters?

Sounds like you are attacking monsters before you are ready. Avoid creatures like Ongr and the giant skeleton. If they come too close, you should fortify that city with your best units, so that if the big monster attacks, you get your best army bolstered with city defenders. Also, putting up walls around a fortress (and a command post) gives your defenders huge bonuses in combat.

Metal shields are a significant achievement, but boar spears might be a bigger one. Get to blacksmithing ASAP, preferably along with Cooperation. Groups (4 members) with boar spears are very versatile and effective additions to your armies.

If you have crystal income, mages are even better. Just get to Enchantment and you can field units with magic staves, which are generally the best ranged attack you can get.

The Terraforming Mod has spells that allow you to turn the terrain into defensive barriers.

First off: no, the AI does not have as much trouble with monsters. On the other hand, those power rankings aren't always a good indication of strength. 26 on turn 107 seems low though. What have you been doing so far?

Different monsters require different strategies: some have special immunities, like the Banshee or Ophidian, some have ranged attacks or powerful spells and should be rushed and taken out asap, while others like Ongr are powerful melee creatures. You can debuff those with spells like Slow and Curse, attack them from range and/or sacrifice some of your weak units.

In any case, always read the description of monsters first. Some abilities can be really nasty. You might be able to take on an Epic skeleton but get Butchered by some Medium undead dudes.

I hunt down weak monsters.  It feels weak and cowardly, but it keeps me alive while I grow.  I keep my heroes together in one army until I have enough troops built to have a sovereign with troops and an army with both heroes and troops.  I work my way up with that first army to having a Medium army rating.  I beeline to Drills to have a bigger army size.  Then, I should be able to clear out a space safe enough to put a city.  I try to keep the borders from expanding and waking up Strong monsters.    After that, I chase monsters internally with one army and clear out city locations with the other.

One of my big changes in gameplay was making my capital a Fortress if appropriate.  Getting troops out sooner and bigger makes life easier for my heroes.  I also use a Sovereign with Heal, which I find very helpful in early battles.

I don't know if it is the best way, but it is the way I have found that woks for me, most of the time.  If I can get that first army up to Medium, I feel I have a chance.

well I'm sorry, perhaps I'm not right but I don't agree.

in my experience the AI settled cities right beside lots of monsters (about 5 in 2 movement range) including insane ones and as soon as I took the city, it took all in all about 3 turns for it to be razed. (couldn't clear those monsters, too high level)

that's what I saw with my very own eyes, I can't talk about the rest

Something else I noticed from your OP is that the two monsters you mentioned, the giant skeleton and Ungr, are random events.  Your monster setting may be fine, just set your random events to a lower setting.

i may be wrong, but I seem to recall those two events happening in a recent game of mine.