How can we kill the deerclops in don't starve together?

In my experience the best ways to kill him is to lead him into a large Beefalo herd, preferably in heat. Lead him into a swamp full of tentacles. Or just plane old kiting with a spike bat (tentacle drop). I've also seen people using Gunpowder. Lastly, if you have enough stingers, make a huge filed of spike traps and lead him into it while kiting.

Usually, when I play as Wigfrid in a server alone, before fighting any season giant, I make sure:

• I can deal enough damage to kill them (at least 4000 damages in the case of the Deerclops) with my equipped weapon. For the Deerclops, that's around 38% of a Battle Spear durability as Wigfrid; it would take around 79% of a regular Spear to take him down with characters who have normal damage multiplier.
• I have 2 or 3 helms available in my inventory, just in case (or anything that gives at least 80% damage protection).
• I have at least 75% of my character's health (assuming you're not playing as Maxwell, since his health cap only at 75).
• I have some healing items (preferably Healing Salve), just in case.

Once I'm ready, I just rush in and attack.

During the fight:

• When you just started the fight, the Deerclops needs to hit you 3 times to freeze you. But if you already got frozen and he unfroze you by hitting you, the next 2 hit will freeze you.
• Everytime the Deerclops hit you, you'll notice your character will get a "pale" tone, which means they're "freezing". Upon being hit twice, run away until the pale colour fades away before charging again, this way the next hit you take won't freeze you.
• If you're playing with a friend, your friend should be standing on the opposite side of the Deerclops, this way you don't both get hit when he attacks.
• Keep hitting him until he's dead.

As I pointed out in the comments, if either of you is hosting the game (rather than playing in a public dedicated server), whenever something goes wrong during the fight, the host can type /rollback in the chat and press enter.

This will allow you to start the game over from the last save (usually the last morning save, or the last time the host disconnected the server), instead of replaying the entire game again whenever you fail and/or die.

Another solution if you intend to fight is to have some Treeguards (not just one) to help you in your fight.

A Treeguard is a tree monster that have a chance at spawning if you cut too many (Lumpy) Evergreen trees. They come in 3 sizes, and their stats (damage, walking speed, attack range, health) scales proportionally with their size:

• Short (they spawn from the tiniest trees, scale is 0.7)
• Medium (they spawn from the medium sized trees and look like adult tree, scale is 1)
• Tall (they spawn from the adult trees and look gigantic, scale is 1.2)

Between the beginning of the game and Winter, you have 21 days, and the Deerclops usually spawn around the 30th day, which mean you have around that much time to cut as many trees as you can to spawn Treeguards. Cutting adult Evergreen will allow you to retrieve 2 pine cones to plant 2 more trees. Repeat the cycle to have more trees to cut.

Whenever you spawn a Treeguard, don't kill him. Instead, lead him far away enough then start planting pine cones until the Treeguard forgets you and plants himself in the ground. Then return cutting trees to keep spawning more treeguards. The more you have the better.

Once the Deerclops attack, if possible, try to make it so the Treeguards are sandwiching the Deerclops (at least one out front and one in the back), this way he can't freeze them both, and one of them is still attacking him.

Upon doing this, 2 medium-sized Treeguards I had spawned, managed to kill the Deerclops "on their own"*, and they both survived.

*: I initially attempted to kill the Deerclops myself, but I had to deal with some shadows (because I had 0 sanity). Luckily for me, when the Deerclops chased me, he broke a tree, which triggered the Treeguards into attacking him. In the end, I barely attacked the Deerclops. So, in a way, the 2 Treeguards sandwiching the Deerclops managed to kill him alone.

If fighting isn't a solution, when I hear a "warning sound" from the giants, I run as far as I can from my camp (if I have one).

Once the giant monster has spawned, I leave him on that part of the map (e.g. a new Bearger won't spawn in the next Winter, if the previous one wasn't killed). You just have to make sure that nobody approach that part of the map (consider it a quarantine area), so he won't notice you nor your buildings.

Although doing that would prevent the Bearger from respawning, I heard the Deerclops despawns after Winter, so he would respawn on subsequent Winter anyway.