Lapras w/ mixed moveset

FB / HP should be a good defender set if Lapras gets a buff or gym rebalance benefits her.

As far as attacking with a mixed set of ice and water there are no types weak to both besides ground. Attacking into Donphan, Rhydon, Golem, etc will hit for SE damage

In general:
A mixed moveset is good for defenders, as it is more likely to hit the attacker at least with one of the moves super effective.
A single-typed moveset is good for attackers, as you chose the setup. A pure water-Lapras is a good attacker against Fire or Rhydon, a pure ice-Lapras is a superb attacker against dragonite.

Having said this, you have a good, hard to exploit defender. Unfortunately, this defender had been nerfed hard and cannot reach adequate cp. See many discussions good-low-cp-defenders versus bad-high-cp-defenders. You are having one of the best low-cp-defenders.