As a Spearman or Dragon Knight, your spear/polearm has some range. Use this to your advantage to prevent getting hit. Try to kill things in 1 hit. Ive noticed that if you kill things in one hit, you can just move on the next and kill it in one hit also. This racks up small amounts of exp from enemies quickly, which is better than taking forever on one enemy that gives slightly more exp. It also saves your aion gold on pots.
Buy the best weapon you can at any time. A good offense, is a good defense. Try doing quests. They give you exp and gold for pots. If you cant do a quest, then get some help. Some people are nice and will help you free of charge.
Try the Zarkum Party Quest. Every time you finish the minimum requirements you get 12,000 experience. If you can get a good group of about 3-5 people in a party, the quest can take less than 10 minutes. Not only is it easy once you get the hang of it, but also if youre careful you will use less than 50 white potions per party quest, which is a lot easier and faster that killing about 90 Jr. Yetis (12,000 experience) for around level 50.
One party quest in about 10 minutes is 12,000 experience, if you keep going for one hour thats 72,000 exp per hour. Thats almost 10% for level 51 in just one hour. Also, if you only use around 50 white potions for every party quest thats only 300 white potions to get 10%, which for me is much better than using not only tons of HP potions but also MP potions, so it saves more money, its fast and easy