Streamline: -10%
Super Mod: Leaf on the Wind: -10%
Regression: -4% for each tier 1 banished, up to -24% with 6
Rare Wild Mod: Winnow: -7%, we don't need this because we have enough

Minimum size: 100% - 10% - 10% - 24% - 7% = 49%
However the achievement only requires 65% size, which is acquirable with just non-wild mods

1. Pick Draft Mode or don't, but it helps a lot, and disable Wild Metamorphosis as it's not needed
2. Decline weapon and shield, pick Research body, reroll if needed
3. Try not to pick tier 1 upgrades we don't need, picking Ataraxia or weapon/shield is good to save a reroll
We can also Banish tier 1 upgrades we don't need
4. Upgrades we want:
Improved Thruster -> Streamline, banish the other 2 in the tree
Rapid Fire -> Warpath, banish the other 2 in the tree
Super Mod: Leaf on the Wind
Efficiency -> any tier 2, banish the other tier 2
5. With Streamline and Leaf on the wind, you get -20% size, so you need -15% more
Each tier 1 banished with Regression gives you -4%, so make sure you have at least 4 tier 1 mods banished before picking it, which should be easy if you followed step 3
6. Swap to Leviathan, reroll/banish/level if needed
7. Achievement is Unlocked, enjoy your tiny noodle