even with disco, you would need another pet with Rune corruption (lowers all resists by 50% for 5 secs), and conductive blast (lowers energy resist by 50% for 4 seconds) to be useful. if they had 90 energy resist, and a barding lower than 160, you would drop it by 28 points. so now it has 62 ER, then halved, 31, then halved again to 16. so it would resist 16% of the damage dealt. so you would do 58 damage, but this is over 5 seconds,so the resists would shift back up (initial 45 damage, followed by5 ticks of 5 damage. so it would be 37, then 3 ticks of 4, then 2 of 3. and thats if it all lined up right).
life leech however really shines when mixed with chiv/ai builds. it costs 5 mana, but heals for 20% of the damage done. mixed in with combat training berserk, you pet takes less damage in the combat cycle, and recieves half healing from all sources besides life leech, while giving them a damage boost.Pair this with a tasty treat and bless, bard mastery. makes them really tanky.