As a defender it's surprisingly fragile, I use one with my miner (duh) along with a pack llama and a pack horse for long mining session, and a troll or earth elemental can kill my three pets without breaking a sweat ...
To defend myself I've gone back to a bit of Magery and Mind Blast (that spell doesn't need Eval Int, just a straight comparison between my INT and the monster's INT), while I invis/stop the pets ... that makes for some really bizarre battles with the miner running circles around the mobs while regenerating enough mana for that last Mind Blast, but it breaks the monotony of mining thousands of ingots.