How do I spend medals

If you plan to run a cav com lineup, investing in Saladin would be better (I assume you gave him a legendary weapon with a better offensive active skill).

Between Saladin and Blücher, Saladin has a -50% archer dmg troop ability and an offensive active while Blücher has Rumour and a defensive aura.

Personally, because I used Saladin in my two runs (both with balanced teams), I prefer him over Blucher who I have never used as a main gen before. I use Saladin as a secondary DPS gen with a strong active to knock out enemies while buffed by Atilla's active.

[Hr)

If you want Alexander as a stalling inf gen, I'd recommend Petain whose aura would benefit Barbarossa or Joan if you want a weaker inf tank and the ability to heal. Plus, both have the 40%(?) dmg reduction troop ability at 6*.
If you want a fort cracker, you could invest in an arty gen with or without aura (like Bismarck with his Destroy troop ability).
I'm not sure whether Alexander can be counted on with his Fury troop ability. You can tell based on a 6* Barbarossa, but the above-mentioned two ways to use Alexander come with replacements that fulfil that role, IMO.