What’s a reliable strategy to play Transmuters in Dungeon Crawl Stone Soup

There are very few guaranteed sources of anything in Crawl. As of DCSS 0.15, the ways to resist electricity are:

• The shock resistance mutation. Great if you get it, but short of being a black draconian (who gets it when his color is declared) or a gargoyle (who gets it from turn 1), it's unlikely. Note that if you have the shock vulnerability mutation, you'll need two sources of resistance to actually resist. The thin metallic scales mutation at level 3 also confers shock resistance.
• If you worship Qazlal, at high piety, you can pray on an altar and receive a level of fire, cold or electricity resistance, or AC+3 (one per customer, offer void if you don't keep up your worship, not valid when under penance). Qazlal can also confer electricity resistance for a few turns if you get hit by electricity while in good standing, but no guarantees.
• Wield a staff of air. A staff of air is a relatively common find by the time you get to Zot. You might carry it as a spare weapon and wield it in case of danger. If you're a melee or ranged weapon fighter or earth spellcaster, this is mostly for running away (perhaps to equip more suitable armor), not for fighting.
• Wear storm dragon armor (a hide is enough). This is often a good option for heavy armor characters, and storm dragons are common enough that you're reasonably likely to collect some armor before Zot.
• Wear, put on or wield an artifact that confers shock resistance. This is obviously up to chance. Note that you can switch weapons and jewelry in one turn; armor requires several turns.
• Quaff a potion of resistance. The duration is 35–74 turns. The Helm card also gives resistance for about a fight's worth at high power levels.
• Take on statue or wisp form.
• The Insulation spell is gone as of DCSS 0.12.

I believe the maximum damage from electric golems and titans has been toned down in DCSS (I think they could one-shot some very-low-hp character builds back in 4.0b26), so permanently-on resistance is less mandatory than it was back then. On the other hand, there are now more shocking monsters (draconians), and there is more risk of needing all resistances at once (draconian packs). I think that in DCSS, electricity resistance isn't really mandatory anywhere for mid-to-high HP characters.

Note that as of 0.15, this is much less of a problem than in previous versions due to the following change:

• Monsters will no longer pick up items that the player has seen. Allies won't pick up items at all.

and choosing Nemelex Xobeh is no longer of any help (it isn't simply piety; you can no longer sacrifice items to Nemelex):

• Nemelex only gives piety for exploration rather than item sacrifices or deck usage.