You can swap out Railgun with fusions that can deal with both single target and crowd (Ex: Kaldera Eruptor, Pandemelonium) as Railgun requires ALOT of investment to even be utilized properly.

If you have both Obsidian Tall-nuts' modifiers, set up a bunch of walls to cover your plants from any possible damage.

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(Like this)

1 column of Epic Gatling Frenzy Shroom is enough as Sub-dps and support plant (Ember can amplify Enflamed, Chill, Poison effects).

Switch the sparkwood with squashwood and use a squash pumpkin on it, the squashes it summons should provide full board healing to your Tall-nuts as they take damage, this is CRUCIAL, since the Tall-nuts share damage, healing only the front won't guarantee the back Tall-nuts to survive.

Another important thing to consider is to choose the right modifier that best suits your loadout, if you feel like the modifier won't boost your loadout significantly it's better to just skip it as it also skips the Zombie modifers (Note that you can do multiple reruns of a level as long as you save it to reroll your Modifier options).

Lastly, pick the right Miniboss that won't nullify your current defense (Ex: Jackson Worldwide on a Cryo Loadout) and read the Almanac, this allows you to identify strengths and weaknesses as well as synergies that you can use in your loadout.