Sounds good. How have the Left achieved this ? Quite a few countries could learn from it.
First of all, Li Andersson, the Left Coalition's party leader was basically guaranteed to be a vote magnet. She is the kind of liked and respected politician that people will vote for as a person, over party lines. With the voting system in Finland (party-list proportional representation), the party as a whole benefits from her votes, she basically earned them two seats in these elections.
But it's also about being a convincing alternative when the party in power messes up their public image. Here is a by now famous publicity photo of the Finns party about Finance Minister Riikka Purra figuratively cutting the budget with scissors with other Finns party members smiling behind her.
This did not go down well at all with low-income voters who due to broad planned cuts in various benefits really started feeling the impact already. There have already been news about families having to move to cheaper apartments, and then we have a photo basically showing the finance minister making light of the necessity of the cuts (and there is indeed a needs to adapt due to the aging population).
Purra even started talking about cutting pensions, which was shot down by the National Coalition party. Really tone-deaf stuff to even start talking about that, especially since pensions are paid from pension funds and have hardly any impact on the government budget.
At the same time there was the obligatory right-wing government tax cut for the rich and no sign of cutting subsidies for large coorporations.
So I guess overall it's about getting your own message to land when people are looking for options, but in a way that is believably realistic.
The Left Alliance managed to outperform even the SDP, the traditionally big left party in this election.