Erda Nova is definitely not worth having if you just want to train and level up. The only practical use for that is to have a second method to cure yourself from being seduced by some kind of boss or something, apart from Hero's Will. Use that node's slot for something else.
Can you 1 shot the monsters that you're training on? If not, it'll help if you include a Chain Lightning/Blizzard/Lightning Orb triad in your V Matrix (with the slot that you freed up by removing Erda Nova, perhaps?) but there are also plenty of other ways to increase your damage to be able to 1 shot.
Definitely focus on Decent Holy Symbol. That helps out so much, especially in the long run. Don't worry about leveling up Ice Age, Bolt Barrage, or Spirit of Snow until you take care of that first, because as the guy above me said, it'll help with finding more nodes while you train, and give more EXP in the process.