Sorry for the late reply. Level 8 seems plenty to get to floor 150, but it really depends on how much EOS you want to spend.
I would suggest you try to get Bone Ring to at least level 4 for each knight (you need only one Bison Horn to craft one ring which makes them really cheap), and then maybe some Diabolic Armor, which is really good for a long time, up to level 2. Weapon-wise just make sure you use axes, bows and staffs rather than swords, and go for the cheaper unique ones like Advanced Dwarf Axe or Titanium Axe. I don't recommend going for the sets.

Also don't forget your pets. If you can, invest 10-15k magic water for advanced pet gacha (if not, go for the normal one). Good pets can make a lot of a difference, and I suggest adopting the ones that give you the most attack.

If you don't want to spend additional own EOS, try to sell some material first. If you happen to find a good unique material like Mithryl or Mithryl Chiffon or Adamantium, sell that and buy the items needed. One Adamantium typically gets you 0.14 EOS which is enough to buy 28 single Bone Rings (at least).