Book 21 is one of my favorite books in Homer's Iliad, because Zeus allows the gods and goddesses to go down to the battlefield and help their favorite heroes. As for battle tactics, as one might expect, when the Xanthus River attacks Achilles, Hephaestus helps Achilles by blasting the river with fire. Thus, fire battles water.
Elsewhere in the battle, Ares' spear is no match for the giant stone that Athena hurls at him. Athena then strikes Aphrodite with "a heavy blow on the chest" (A.S. Kline translation).
Poseidon challenges Apollo to fight, but Apollo decides that it would be unwise to fight with his uncle.
Artemis chastises her brother Apollo for not fighting with Poseidon, but then Hera snatches Artemis' bow and quiver away from her and "laughing all the while, boxed her on the ears with the weapons as she writhed."
This left Hermes to do battle with Leto, the mother of Apollo and Artemis, but, after seeing what Hera has done to Artemis, decides that fighting with women who have mothered children for Zeus is a dangerous thing. Thus, Hermes tells Leto that she can tell everyone that she defeated him:
Go boast to your heart’s content to the immortals of how your great strength bettered me.
Thus, the battle tactics of the gods in Iliad 21 are relatively non-existent, other than Hephaestus' battle with the river and Athena's battle with Ares and Aphrodite.
Apollo as usual sides with Troy. He deceives wroth Achilles so the Trojans would be able to escape and find shelter behind the walls. But Pheobus would not have intervend hadn't Vulcan protected Achilles against the flood of the river. So,mainly, you have the intervention of two gods: one on behalf of the Achaeans: Vulcan. The other on behalf of the Trojans: Pheobus Apollo.