Austria starts as HRE Emperor and has one of the strongest mission trees in the game — it can snowball into a world conquest if you play it right. First things first though: fill your forcelimit. You'll need a full army before picking your first fight.

For idea groups, the recommended order is Diplomatic, Influence, Religious, Administrative, Humanist, and Offensive — swap Humanist and Offensive depending on whether you want to lean into the HRE vassal swarm or not.

Your first war target is Bohemia. Rival them early for the power projection bonus, then use the Enforce PU casus belli to bring them under your crown. Call in your allies — you've royal married Castile and allied the electors by now, so this shouldn't be a hard fight. One war won, one junior partner secured.

Unlike the Brandenburg to Germany run where you conquer provinces directly from the start, Austria's early game is built around establishing personal unions rather than direct annexation. Your first two are Bohemia and Hungary — Bohemia through war, Hungary through the Archduke of Austria / King of Hungary event around 1455. If the event doesn't grant you the union for whatever reason, you'll need to go to war for it too — call in your allies, Burgundy especially for a good measure. Two crowns either way, no need to paint the map just yet.

Milan is your third personal union, handed to you through a scripted event. The war itself is straightforward — use it as an opportunity to hit some Italian minors on the side if you're working toward the Italy missions. Three unions at once is the requirement, and at this point you should have all three running simultaneously.

By now you likely have a small OPM subject sitting around — Serbia, Bosnia, or similar — that's ready to be diplo-annexed. If you've played Austria before you've probably already popped this achievement in a previous run. Either way, this is a natural checkpoint to clean it up if it hasn't triggered yet.

The Reformation is the biggest threat to forming the HRE, so crush it fast. When the first Center of Reformation spawns — usually in a small nation, often an OPM — declare immediately even without a casus belli, as long as you won't trigger a coalition. Enforce religion and the center is gone. The next ones tend to spawn in Brandenburg, Denmark, or Sweden. Attack, split off the capital province so it holds the CoR, enforce religion. Repeat for all six centers, both Protestant and Reformed. Your empire is large enough by this point to handle it.

Completely optional, but why not have some fun. By this point there should be weak Indian nations in the Bengal area willing to sell you a province for a few thousand ducats. Use the Charter Company diplomatic option, invest in the District and Military Administration, and tick the achievement. Nothing critical, just a detour if the opportunity is there.

Bohemia, Hungary, Milan, Burgundy, Castile, Poland, Lithuania as personal unions. The Reformation crushed. France weakened and split. Italian princes absorbed into the Empire. Once Renovatio Imperii is done and the reforms are through, the HRE becomes a real centralized state and the achievement is yours.