Do you separate those rolls in your mind (r-at, st-art) or try to see them as one motion?

In general, the most tricky movements are limited by the lack of independence of the weaker fingers. Piano etudes will help, if you have a piano. I don't know of any typing etudes of a similar nature, designed to work rigorously towards the independence and strength of every finger. Well, some typing courses do that but I still think that the piano etudes are more rigorous about it since claviatures for music have been around for several hundred years more than 'typing claviatures'.

Suddenly it became extremely difficult to type 'contrast' many times quickly. Gah! It's your fault!

Mostly I separate the rolls in my mind. When I type I think i-on or r-at thinking of always trying to do some kind of inward roll in my head when most important. I don't own a piano or a music keyboard, so I've never any finger strengthening practice. Colemak is so nice and on the home row, but these rolls that are just impossible, it just like tangles up my fingers. What I've been trying to do is practice typing common words like that to see if I could improve my accuracy because that is mostly what is getting to me.

Here's some words to use for training rolls, which I understand to be three or more (i.e., four) keys going in one direction.

First, simple rolls, i.e., no gaps inside the roll.
Left hand: >arst>, >ars>, >rst>, <ars< (=sra).
Right hand: <nei< (=ien), <eio< (=oie), <luy< (=yul), >neio>, >nei>, >eio>.

=== arst:
barstool
Barstow
clearstory
earstone
Hearst
Karsten
starstruck
tearstained

=== ars:
arsenal
avatars
cars
coarse
dollars
harsh
Lars
Mars
marshal
narsarsukite
parse
radarscope
Sears
sparse
stars
tears
varsity

=== rst:
afterstate
airburst
airstrip
Amherst
Bakerstown
burst
cornerstone
doorstep
first
hairstyle
overstate
superstar
thirst
thunderstruck
worst

=== sra:
crossrail
Disraeli
disrate
Israel
lebensraum
transrational


=== ien:
alien
ambience
ancient
audience
bien
client
Dienbienphu
fiend
friend
gradient
hygiene
lenient
O'Brien
oriental
patient
quotient
sapient
science
sienna
Vienna

=== oie:
joie
Savoie

=== yul:
embryulcia
Yulan
Yule
Yuletide

=== neio:
gorgoneion

=== nei:
Brunei
cuneiform
gneiss
neighbor
Neil
Schneider

=== eio:
meiosis
pleiotropy
semeion

Then, rolls with a gap.
Left hand: >ar-t>, >a-st>, <ar-t< (=tra), <a-st< (=tsa).
Right hand: <ne-o< (=oen), <n-io< (=oin), >ne-o>, >n-io>.

=== art:
apartments
art
artisan
artist
bartender
cartoonist
darted
Earth
farther
garter
heart
heartease
heartened
heartsoreness
heartstring
mart
nonpartner
part
partridge
restartable
smart
start
Stewartstown
Stuart
sugartree
tartar
tearthroat
thwart

=== ast:
abreast
aftertaste
alabaster
Anastasia
astrograph
bastard
castrate
catastrophe
contrast
devastate
distaste
fastest
hasta
intrastate
pasta
pastoral
postmaster
rastafari
sarcastic
toasters
ultrafast
vast

=== tra:
abstract
Alcatraz
Amtrak
astral
astray
attract
betray
central
citrate
contra
contraremonstrant
distract
entrance
extra
extraspectral
intra
intratracheal
magistrate
mantra
neutral
outrank
penetrate
retract
straddle
straight
straw
tetranitrate
trace
track
trade
traffic
tragic
trail
trainee
tramcars
trans
Trans-Australian
transrational
trap
ultraoutrageous

=== tsa:
flotsam
jetsam
outsail
portsale
tsar

=== oen:
bioengineering
monoenergetic
Phoenix
subpoena
toenail

=== oin:
Antoine
appoint
boing
booing
coin
coincidence
coining
coinvent
conjoin
doing
going
heroine
join
knee-joint
oink
ointment
ongoing
point
proinclusion
shooing
viewpoint

=== neo:
Borneo
geneological
Neocene
neocolonial
neon
neoprene
spontaneous

=== nio:
acrimonious
Antonio
antonio
crinion
cybernion
dominion
harmonious
ingenious
junior
minion
niobium
nonionized
non-union
onion
opinioned
senior
union

Of course, there are thousands of words that could be used, I just picked the ones I liked. And while they're not listed as such, there are words where the letters from the roll occur directly before or after the roll, e.g., "start", as mentioned by the OP, "tearstained", "avatars", "lenient".

Now the words just need to be put together into one or more lectures of reasonable length. Doing them in alphabetical order is probably not very entertaining :-)

Warcraft, yes, hehe. "Troll on the bridge!" :-D

Individually, the combos 'war', 'cra' and 'ft' aren't so bad but put together in one word they do become fatigueing. Not sure what to do about it.

I don't see a problem with 'heartbeat' though? I use the Curl(DH) mod that improves the HE bigram; maybe that has something to do with it?

There was a "worst words with Colemak" topic a while back. Lots of ugly finger trolls there. :-)

But fortunately, they're rare. That's what makes a layout good: Making the common stuff better, and the ugly stuff rare.