20 is the big one in the field with the canons, yeah?
Sprint northeast to the first canon, sprint through it, then enter the town to the right of it. Buy the horse, heal. On the horse head back east knocking out as many cannons as you can (there's 5 IIRC) and then take the tunnel to the north, sprinting the entire way. Hit two more canons on the north side, which should bring you to level 24 or so, go into the town, buy the lance, heal, buy a leaf (I think there's one there), then rush the boss.
That one took me a while to get the run down properly. It also helps if you equip stuff that boosts your attack and prevents you from getting knocked back beforehand to help with the boss.
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i'm dumb as hell holy crap -ExThaNemesis