when a person with dementia becomes "combative", it is a sure sign that we caregivers need to look at ourselves and how we interact with our person. i'm not talking about the wild and crazy all night long rampaging person( who, by the way, is much more likely to be schizophrenic, psychotic or have PTSD)
people with dementia don't want to be bossed around (unlike the rest of us?), argued with, made to feel inadequate and childish. when they get mad at us, we've usually done some or all of those things. to manage the situation better, we caregivers need: a) to slow down; b) to be respectful; c) to make suggestions, not give orders; d) to keep things simple and do them in step-by-step order and e) be kind, be kind, be kind.
when they react badly, we need to ask ourselves, uh oh what did i do here? if they hit out, we have frightened them, hassled them, moved into their space instead of stepping back out of it, and probably tried to hurry them too much.
if it's not your approach that scares them, maybe it's what you're trying to do. most people with dementia are scared by the shower and there are many better ways to deal with that than trying to insist on it. see "Alzheimer's 911" by Frena Gray-Davidson (me) which concentrates not on disease but how we learn to communicate well with a perosnd ealing with dementia. remember, you may tell yourself it's worse to be a caregiver than to have dementia, but that is NEVER true.