Until you're level 14 to use spider mines, put 1 point or in blast canon or in flame hammer and every 3 levels, before spending your next skill point, remove the blast cannon/flame hammer, level some skill, then put the point back at blast cannon/flame hammer. If you want to use it, you may take all the skill points from blast cannon, heavy lifting, coup de grace and put these points at spider mines 15, bullwark 15, dynamo field 10 and tremor 15. Also, I believe blast cannon is more fun. Some people love spider mines, and it's really an overpowered skill, it's only flaw is the fact that you can only get it at level 14.
There's 15 skill points left for you to spend.
