My mother and I used to argue to get her into the shower. She would get mean and say nasty things to me. I'd be upset for hours after. So, I contacted the office on aging and set up aides that come to my house and help her shower 3x per week. She sometimes resists at first and tells the aide she is "sick" but they get her to shower and she doesn't yell at them like she does at me. Good luck! (And she HATES to remove her panties. They could become glued to her skin if I let it go.)
I was having the same problem with my mother. I moved her in with me after her stroke and gave her the bathroom with a walk-in shower. I put a bench and hand held shower hose in it thinking that would be the safest and easiest option. She was only showering once a week and granted she never sweats but has occasional incontinence, so I would remind her the day before if we were going to the doctor or something. I found out this winter she gets to cold, she I reminded her there is a heat switch in the light\exhaust unit that will blow down hot air which helped. But what I found out when she went to stay with my sister a couple weeks is, she prefers a bath, she was hopping right in her tub, but couldn't get up or out in it without help. So now I'm in the process of purchasing a bath lift, I showed them to her online and her eyes lit up. I'm going with that over the the walk-in tub mainly because they seem safer (no standing in a wet tub), they're portable, less expensive, and will lower her within 2 1/2 of the bottom of the tub (so you use less water. I worried with the walk-in tub if you're sitting on a raised seat how much water it would take to fill, if she slipped she could drown, also you have to sit there until the water drains out to open the door so she'd get cold. I don't know if this helps, it's hard when they won't\can't tell you what the problem is andare unable do the things they use to.
