Work doggedly or persistently
Keep one's nose to the grindstone
Keep one's shoulder to the wheel
Peg away
Plug away
Work
Slogger
Strike heavily, especially with the fist or a bat
Slug
Swig
Hit
Walk heavily and firmly, as when weary, or through mud
Footslog
Pad
Plod
Tramp
Trudge
Walk
Slosh
Splash
Squelch
Squish
Splosh
Slop