Also I'm a casual so I just want something that's challenging enough I have to try a little but not so challenging I have to know all about IVs and EVs and breeding and whatever the else heck there is to be good enough to win without struggling a lot/spamming items/grinding/whatever
Let's put my information above into perspective: If you have a Lv 15 Squirtle, these are my approximate calculations at Lv 15...for those more technically inclined, this was done with 15 IVs for Squirtle. For everyone else, this means it's just a statistically average Squirtle:
0 SpA Squirtle [Water Gun] vs. 0 HP / 0 SpD Steelix: 14-18 (26.9 - 34.6%) -- 0% chance to 3HKO
0 Atk Steelix Body Slam vs. 0 HP / 0 Def Squirtle: 17-20 (42.5 - 50%) -- 0.4% chance to 2HKO
Which, put in English, would mean that the average Squirtle will take less than half its HP per attack by Steelix (which, by these rough calculations, is under 20 HP damage), The Squirtle will take down the Steelix in about 4 attacks. It's much harder than the Onix I modelled this after where Onix was a one or two hit kill (90-120% damage).
This is the most favourable match up against Steelix and you'd likely require level 16 to beat the first gym. In fact, a Level 16 starter, regardless of the typing, would trounce Steelix at this point (due to the stat boost of evolution) in about the same ability as a Level 15 Squirtle...and a Wartortle would turn it into an older style Gym battle, destroying the Steelix in a straight up fight.
That's kind of what I hope Pokemon would get to. A fight that is difficult and intentionally so. But one that you can obviously handle/struggle your way through.