In succession, key veteran halfback Ricky Alabaster limped off, meaning career hooker Jack Yarrall had to cover that spot, while powerful second-five Josaia Bogileka came to the sideline with his arm hanging.
Other players had to stay on or come back after knocks, while young centre Jamie Robertson was also helped off in the second half.
Now doing all the play-making by himself, veteran Marist first-five Sean Brown relied on kicking for territory to try and get his team out of trouble, but Ruapehu's back three seized their opportunities.
Barnstorming winger Royce Trow fed off both his loose forwards and midfielders to run in a hat trick of tries, while the season's leading points scorer in fullback Mitchell Millar had another profitable day with an 16-point haul.
Reserve winger Corey Carmichael impressed after coming on before halftime, while centre Tautahi Rawiri cut loose in the final quarter with a depleted line in front of him.
Up front, hooker Roman Tutauha made some punishing tackles, ably backed by both Hakaraia brothers in Gabriel and Te Uhi, while skipper and MVP season points leader Jamie Hughes was again dominant attacking on the fringes.
It was only after those aforementioned players were subbed, with the game safe and a semifinal with Byford's Readimix Taihape looming, that Ruapehu let the foot off the gas, as Marist were able to turn the scrum against bench players a couple of times to get turnovers, while driving over for two consolation tries.
However, successive dropped passes from their locks on the carry from the second or third phase meant Marist could not capitalise further, and Ruapehu immediately spread wide into the big gaps, only just missing out on raising 50 points as Trow was hunting his fourth try just before fulltime.
As well as Yarrall and Brown, Marist's best was prop Gordon Karaitiana, taking on added responsibilities without Tofa, while Cody Wickham gave his all from No 8.
But with several men playing through pain, Marist didn't have the firepower, as Ruapehu's No 8 Campbell Hart and second-five Troy Brown busted the line open multiple times in the final quarter.
"The second half of the season has been tough on the boys," said Marist coach Marty Bullock.
"They didn't give up, playing on with injuries – Brad Graham had ten stitches in his leg.
"We were running without a backline at times.
"We're disappointed to finish the season."
Marist had to go down "murderers row" in the second round by playing all the other semifinal contenders consecutively, and after his first season in charge, Bullock was asked what they needed to get back into finals rugby after missing out these last two years.
"Pre-season is really important, we need the boys turning up in January and February [by] doing their own physical fitness," he said.
"It's depth as well, we might have to do some more recruiting. It's getting tougher and tougher in the clubs to find players."
Ruapehu co-coach Kim McNaught was happy with his side's effort in a game which, on paper, could have still been quite difficult.
"We knew it was going to be tough - last year we lost [to Marist].
"We had a focus to come down here and get the result."
Ruapehu have had a tough road schedule in the second round with only one home match, but McNaught felt they had weathered the previous month-and-a-half quite well in the buildup to the playoffs.
"Injury wise, we're all good," he said.
"It was hard with the breakup for the [representative] trial. But we're trucking on pretty good.
"It's a shame we can't have an all-Northern [sub union] final, but we're looking forward to it [with Taihape]."
Veteran flanker Andrew Evans pounced on a chip kick which young Marist fullback Caleb Crosse let bounce to set his team on attack in the opening minutes, and two rucks later lock Jackson Campbell was diving under the posts.
Marist kept making mistakes in the back field, and then were penalised for holding while trying to drive out from their 22m, with Millar adding the three points from in front.
A Hughes hit-up from a 15m lineout set Ruapehu up again, and although Millar bobbled the pass, he stayed calm to reclaim the ball and place it down in the corner despite the cover defence.
Marist tried to regain some territory, but mistakes again cost them, and Ruapehu showed them how it's done as after a succession of strong carries by the forwards, Hart went down towards the right-hand corner again and drew in three defenders before offloading back inside to Brown, who got the ball down in traffic for 22-0 after 34 minutes.
Given their circumstances, Marist might have been able to handle that if they could get to halftime and have the slight breeze behind them, but Ruapehu weren't done as Hughes linked with Carmichael and Rawiri to have a crack at the line, before they recycled and Carmichael dived under the tackler to score, with Millar again converting from out wide.
Another Ruapehu turnover inside Marist's half set up a series on attacks on both sides, before Hart linked with Millar and then the ball was transferred to Trow to scoot in at the corner.
Hughes departed and Ruapehu eased off slightly, letting Marist's forwards get into the game as following a series of hit ups, their skipper Graham was pushed over the line.
With play becoming pretty scrappy both ways, Marist drove from a penalty lineout and after Karaitiana forced his way in-goal but with bodies underneath him stopping the try, he recycled back and fellow prop Cameron Neilson was very clever to pick up the ball and just push it forward to the partially exposed white line.
But 34-10 was the end of Marist's rally, despite competing well in the scrums, as after Carmichael made a couple of probing runs, across field halfback Kahl Elers-Green ran through a big gap and sent the ball to Trow to run in at the corner.
Dropped ball from the tall men like Brad O'Leary and Hamish Broadhead kept hurting Marist, and then Rawiri ran on to a great wide pass to hit the gap and offload to Trow for his hat trick with the clock ticking down.
Ruapehu 46 (R Trow 3, J Campbell, M Millar, T Brown, C Carmichael tries; Millar pen, 4 con) bt Marist 10 (B Graham, C Neilson tries). HT: 29-0.