Shyvana is a fantastic user of Press the Attack thanks to her ability to instantly proc it with an Auto->[Q] reset. The instant exposure allows Shyvana to duel and shred champions effectively.
Triumph on Shyvana allows you to dive into the back line, absorb the brunt of the damage for your team, and heal for a ton from any kills or assists to keep fighting.
Shyvana's bonus resists from her passive further empowers that health she gets back.
More Attack Speed means a faster/healthier jungle clear and more dueling potential in early skirmishes.
WhyCoup De Grace
Coup De Grace is the go-to tune for reliable damage in fights where you're able to get to the back-line and shred the enemy squishy champions.
Celerity on Shyvana help you to overcome her biggest pitfall of being kited. The extra bit of Movement Speed synergizes well with Shyv's [W] and especially well with Waterwalking when skirmishing in the river.
Waterwalking on Shyvana further empowers her desire to power farm the jungle and clear the entire map.
The extra bit of AD also allows you to take early objects (Herald and Dragon) on respawn, empowering your passive even further.