Slow speed front lines get destroyed in top tier PvP, that’s the way it’s been for quite a while. If you’re having trouble with a particular enemy FL then either accept it’s a weakness of your FL or create a FL which counters it.
That said, FL combos which decimate your whole front line are quite brutal and annoying to build against because you either need specific monsters or to use things like camouflage or hold ground monsters, which massively limits your FL strategy.
EDIT: Just seen this other thread which is the same. I’m closing this one, discussion should happen in that thread.