There is a trick you can use to quickly level up LRs. Make a new team just for leveling up card(s). In this team, make sure to have a card that says battle EXP up at least 200 or something along those lines, even better if you have two. These cards make the cards you want to lvl up level up even faster.
For instance, I have GSR Lirissa and her 2nd skill says battle exp +200 so I can use her in a level up team to level up the LRs that I want to level up faster.
You can use level up teams in duels, which will help you quickly lvl up your LRs.
Then, you should pray to RNGeseus that you get Athena from ultimate summon. At max level her skill increases battle exp by 80%. 80 plus 20 is 100 so 100% is still enough to increase LRs quite a bit, not as much as 200 though but its a start!
And sometimes the SRs in abb ring exchange have battle exp +200 as their awoken second skill so who knows? Maybe the next abb sr will have the second skill in gsr form and you can get a good EXP booster from there