After getting to around Lvl 33 I tried a few different methods but everything was taking too long. I believe this led me to find the fastest way to gain EXP. Even up to Lvl 49-50 this only takes about 10-15 mins for one level.
You simply need to start a new campaign on Insane and speedrun up to the point where you find Claire in a blue coloured room. I think because there is so much fresh map to explore and storyline/bosses (which I'm assuming count a lot more towards EXP than just regular kills) that this leads to your level shooting up.