• vayasā-atulyaḥ — unequal in age
  Instrumental + adjective. Arjuna is younger than Jarāsandha — kṣatriya code holds the elder should not fight the much-younger. The technicality saves Arjuna for Kurukṣetra.
• tulya-balaḥ — equal in strength
  Compound nominative. Bhīma alone is equal in mace-strength. Jarāsandha's self-knowledge is accurate — he is the strongest mace-fighter alive — and so is his choice.
• bhīmaḥ — Bhīma
  Nominative. The chosen opponent. The verse marks the moment Jarāsandha walks into Kṛṣṇa's plan thinking he selected it himself.

• Whom am I currently underestimating?
• Where does taking someone seriously change the encounter entirely?

Jarāsandha rejects two opponents and chooses one — like a king who measures swords before a duel. He is not stupid; he is precise. But his precision still misses that Kṛṣṇa is the one who chose Bhīma already, and that the duel is already lost.