For 25-29, go to Cianwood City and surf for Tentacruel & Tentacool. They give decent experience and are relatively easy to take out.
For 35-38, you need Waterfall to train any faster. If you have Waterfall already, go to Route 47, go down to the bottom level of the cave, surf, up the waterfall and there's a small patch of grass with stuff like Miltanks and Dittos. Let the Dittos transform if you're confident you can take whatever it transforms into out in 1 hit. If not, just go for the Miltanks and/or go there at night so there's Noctowls to take out as well.