The reason why he has problems defending himself against electricity is that there aren't exactly many organic materials that could provide him with resistance to it. Amber was literally the only material I could come up with. And even that is more plant-based than animal-based.
Like you, I wonder about this as well.
if he could transform parts of himself as some metallic substance, like the edges of his fingers or tails, I bet he can make a sacrificial inoperable umbrella limbs that can supposedly catch/deflect stuff (preferably gigantic ones, imagine Met bouncing off of it instead of going through mc when he does one of those cool dash moves he does lmao). maybe make a huge ass metallic limb, impale it in the ground, then cutting it off to make a sort of ground. The problem is the speed that limb deteriorates into sludge after getting cut off.
if the mc instead goes fully metallic, there's a melting point, so the fight would have to be in the ground, but normies near that ground = dead. lightning striking ground has no visible damage usually other the burnt stuff but that shit's charged, like an electric minefield.
if mc could become some rubbery mf, could work, like water blasting at a sealed container. instead of taking shock and burn damage, you get physical damage. if it's indra doing the blasting, you're just gone as we can assume he's pushing whatever millions/billions and above volts there are.
ps
I always liked Met's power. it's one of the most visually "cool" powers to have in any media. He can do really fast strikes that wouldn't visible to the eye (but is incredible loud, breaking the sound barrier and all that) and enemies are just cut in half or there's holes in em, like a tree vs lightning, fire is just an after effect, it gets split in half first. oh anyone caught in the AOE is probably either deafened or tossed aside cuz those sonic booms. Tiffany's still faster tho and can actually do stealth at day time lmao