How do you raid tombs in gemcraft labyrinth

The best way I've found is to only bomb the tombs on a level with mana shards until you've got lots of skills. The best way to ensure victory is to put a few towers with grade two gems in a position where they will only shoot the mana shards, but be sure to have enough fire power to kill the monsters. Then, on the last wave with only one or two monsters left pause the game and gem bomb the tomb to open it. If you have gathered enough mana from the shards you should have quite a few grade 6 or higher gems. I wouldn't open the tomb unless you have at least five grade 6 or higher gems. The easiest fields for this are L3 and L13.