"prevents him from changing form and reverts him back to his black slime form" is not really accurate
Well, there is of course a difference between a taser shot and the concentrated energy of several lightning bolts.
It's like cell breakdown and explosive cell breakdown.
This is why rubber is relevant, rubber is not conductive but even if he transformed into it as soon as it came in contact with eletricity it would explode anyway, so a layer could stop the eletricity from passing deeper (in theory) but the layer would explode, not the most safe strategy even if could potentially help a little.
It is not relevant, because even though rubber is non-conductive, it is definitely quite vulnerable to electric energy.
Or do you think that if you put a paddling pool over your head during a thunderstorm and got struck by lightning, you would be completely protected and escape unscathed? Concentrated electrical energy also carries a lot of thermal energy. Just for fun, the temperature of normal lightning is 5 times greater than the temperature of the sun's surface. The rubber would melt faster than butter after being hit. Not exactly ideal protection
I think that hardened amber would be ideal for MC, as it is a great solid and absolutely non-conductive element, which is technically an organic substance. Armor covered in amber plates would provide extremely effective protection against electricity. And even if MC wasn't able to create amber himself, Deryl could help him.
Last update Michael comments on how he is receiving instructions regarding his growth in dreams, Mc compares Michael's "trial" to evolve with his own "memory paths" since they unlocked his memory powers further, we can especulate that Ella didn't have her eyes at first evolution or that she had but they grew more powerful after some trial.
Given the flashbacks in the last update, it's quite possible that Ella is a completely artificial superhuman, created using a series of experimental drugs. For now, it is pointless to debate when she unlocked the power of her eyes. But it was definitely before lvl 5 and highly likely before lvl 4. So probably sometime between 1 and 3.