Seeing the net not quite work, Silenus decides to leap off and try to get on the back of the dragon. That's got to distract it.

As he leaps, his own exuberance gets the better of him and he feels his inherent magic bubble up inside.

(Bonus Action - Enter Rage - Wild Surge Roll: 2  ; Action - Leap on to back - Athletics 16  )

While that extra energy did nothing to help him actually land on the dragon, he does still have a protective aura around him (Whenever a creature hits Silenus they take 1d6 force damage as the magical energy pushes back against the attack).

Rinn's Strength Check: 3

Rinn fires a [Tooltip Not Found] at the dragon.

Longbow Attack: 13 Damage: 7 (Extra roll if advantage: 17)

Sneak Attack if applicable: 15

He then moves position to some new cover and hides again (bonus action, stealth: 22).

The net doesn't find purchase, Rinn's unable to pull it back. Silenus' attempts with the harpoon miss. Silenus leaps from the top of the keep for the dragon as it turns to face them. He's just off kilter enough to miss the dragon's back. In its turn, it snaps at Silenus attempting to bite and graple. 

Bite. 29 to hit: 15 piercing damage plus 3 cold damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 14 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

The dragon misses Silenus. 

Silenus takes 9 falling damage, reduced by half for rage, landing just short of the plummet of the revine. 

The dragon arcs around for another attack. 

((Silenus and Rinn new turn.))

Silenus WIS Save: 19 (rolled on game log)

Silenus shakes off the fall, looks at the dragon for a moment, wondering what it's trying to do. Being slightly dazed from the fall, seems to have given Silenus a bit of help in resisting the fear the dragon was trying to cause.

He races up close to the dragon, swinging his great axe as he does, hacking at the dragon once, and then again, before leaping upwards as high as he can to try and get on the creatures back.

Attack 1: (16+7) = 23 to hit, for (7+4+2 from rage) = 13 slashing

Attack 2: (2+7) = 9 to hit, for (10+4+2 from rage) = 16 slashing

Movement: Jump (3+4 from STR +8 from Mirthful Leap) = 15' vertically up

Dragon Passive Perception 21 Check 30

Rinn's Wisdom Save: 21

Rinn shrugs off the fear from the dragon and shoots another +1 arrow at it.

Longbow Attack: 15 Damage: 7

Sneak Attack if applicable: 10

Dragon breath recharge 5

Silenus is fifteen feet in the air, Rinn's arrow doesn't pierce the dragon's hide.

It releases a torrent of ice breath catching both of you.

Make a DC 19 Constitution saving throw, taking 46 cold damage on a failed save.

On a success take half. 

Then snaps at Silenus (bite 12 to hit, 9 piercing damage plus 3 cold damage.

And swipes its tail at Rinn (Tail. 11 to hit, 15 bludgeoning damage.)

Silenus make a strength saving throw DC 18

(( Silenus halves the piercing damage, so 7 HP ))

The white dragon receives 3  force damage (from wild surge) when it bites Silenus from the magic surging back.

(Wow DC 19 - Silenus Con Save: (19+7) = 26 )

STR Save: 11+7 = 18

Silenus manages to get himself out from the jaws of the dragon, the wild surging magic helping push him free. He shivers a little at the cold, then angles himself around the white dragon and hacks away with his great axe again

Attack 1: 11 to hit for 18 (incl rage)

Attack 2: 21 to hit for 7 (incl rage)

Rinn's CON Save: 11

(( If this doesn't save, then Rinn is down - not dead, but only just. ))

(( Silenus rolled a 19 CON save? That's lucky! ))

Silenus takes 31 HP cold damage and 4 from the bite.