The GUINNESS PRO12's second-placed team sit third in their tight European pool behind Toulouse and this weekend's opponents Bath.
Only a win will do against the English side, and Gregor Townsend's men could still pay the price of failing to secure the bonus point against Montpellier if they fail to go through as a best runner-up.
And van der Merwe - who grabbed a hat-trick in the 21-10 win over the French side - knows Glasgow will have to recreate the form that saw them beat Bath 37-10 earlier in the competition.
"When we go to Bath for the last pool match we must improve on what we did against Montpellier in the opening 40 minutes," the 28-year-old told the Daily Record.
"It is always a positive when you beat a top team from the French Championship - and it is always nice to pick up tries.
"But on that occasion there was definite disappointment because we couldn't get that important fourth touchdown for a bonus point.
"At least we did win - and that means we have given ourselves a chance of going through to the quarter-finals this weekend depending on what happens elsewhere.
"Our big problem was our mistake tally in the first half when we maybe tried to do too much too soon, and couldn't hold on to the ball at critical moments.
"We were better after the interval but will need to improve against Bath to get the win we need."
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