You do know that any advice comes with a price tag of pigtures of the DespairPigs ? 
Bathing guinea-pigs does not win their friendship . We bathed ours before they went to boarding, when we bonded new piggies or a bum dip if needed . They sulked afterwards .
30 years ago I used to have a Peruvian guinea-pig, I kept her fur floor length and shorter round her bottom. (I didn't know she was Peruvian when I got her ). I think back them you got Smooth/Abby /Peruvian . Now there's dozens of breeds .
Our last piggie (who died in April this year) had long fur and DD last boar was a Teddy ( springey but tended to be dry skin)
We found rubbing any matts with coconut oil before teasing/combing out and pre-washing the Teddy with coconut oil melted in warm water helped .
I didn't brush but found a fine baby comb or plastic nit comb (the double sided one) worked well.The prongs are short and blunt but fine
If you need to trim ,hold the fur between middle /index finger next to the pig so you protect them in case they move . Then snip into the manky bit
Do a bit at a time especially if your boar isn't happy being handled . Can you towel wrap him, leave his rump sticking out ? He might even accept the grooming as cuddle time - parsley works well to bribe them
We kept GP7 fur quite short round her bum (with a comb-over !) to reduce the risk of flystrike . They love to sit in their own pee/pooh don't they .
When our teddy boar was on his way to The Bridge we bathed him every day and rubbed a bit of coconut oil on his legs to protect him from pee scald (and he stank ) we wanted to keep him comfy but not stress him. He didn't mind the bum dip and loved the cuddles in the warm towel after .
Gorgeous Guineas has some lovely shampoo , we got Posh&Go for the conditioning effect ( I stole the coconut oil/water idea from their 'Melts' )
Smooths are so much easier 