If you're having trouble getting past the Magma layer, make sure your Dojo upgrades are maxed out first. Relic powers also help a lot, so try to unlock those if you haven't already. Then use your best pickaxe.
For the darkness, keep your torch on at all times in the Magma layer and use Mageflare Runes every 7-8 digs down. For the Burn debuff, Winterheart and Magmawalker boots are both very helpful if you have them. Hope this helps!