[RPG] Is it possible for a Druid to defeat a Wizard?

For your starting class, I would recommend Barbarian. You'll get a bit more health than the druid, and you'll also get better saving throw proficiencies, skill proficiencies, and armor proficiencies.

To elaborate on saving throws, Barbarian gets STR and CON proficiencies, while Druid gets INT and WIS. Generally you want to prioritize saving throws which will protect you from instant-disabling effects; things that will immediately take you out of the fight. CON and WIS are probably the most important in this regard. CON is the most common save in the game. It covers poison damage and the poison status effect, but more importantly, stun, paralyze, petrification, death, blindness, life drain, and long-term diseases and curses. WIS covers fear, charm, and mind control.

If both these saves are important, then why do I say the Barbarian's saves are better? Because STR saves are far, far, far more common than INT saves. STR saves come into play from a variety of enemies, usually for resisting prone, grapple, and knockback, which will be important for a meat shield. INT saves, on the other hand, are extremely rare, only used by a tiny select few monsters, mainly mind-flayers and their kin. Therefore, while CON and WIS are roughly equal priority, STR will give you much more benefit than INT.

Regardless which you choose, it would be a good idea to take the Resilient feat for whichever of CON and WIS you don't get proficiency in. This may impact your leveling order if you feel you desperately need this feat, but probably shouldn't make too much of a difference early on.

As far as the starting proficiencies go, Barbarians give you all martial weapon proficiencies, while druid is more limited. Armor proficiencies are the same, although druids cannot use metal. This may be a significant caveat for this multiclass combo if your DM chooses to enforce it.

For skill proficiencies, the Barbarian skill options relevant to you are Athletics, Intimidation, and Perception. Athletics makes you better at resisting grapples, which is very important for a tank. Intimidation might be a good option if your DM lets you use it in combat, and lets you use STR for it rather than CHA. Perception helps with group perception checks to keep your group from getting surprised. For Druid skill proficiencies, Perception is really the only skill relevant to a tank. There isn't much in the Druid skill list that's important for your optimization goal.

As for continuing progression, you'll want at least three levels of Barbarian for Bear totem resistance to all damage. If tankiness is your only goal, then the feat/attribute boost at 4th level Barbarian is tempting, but there isn't much else in the way of defense available in the next few levels. At most I'd say to max out Barbarian at level 5 in order to pick up the Extra Attack and +10 movement while not wearing heavy armor. After that, you'd probably be better off going druid.

CONCLUSION: Take one level in Barbarian to start. From here you can either go Druid 2 for Wild Shape, or continue on to Druid 4 to get your attribute boost/Feat as soon as possible. After that, return to Barbarian and continue to Barbarian level 3 or 5, depending on how much you want the extra feat, extra attack, and movement bonus. Once you're satisfied with Barbarian, max out Druid.

Use enemies with alternative damage types

The lycanthropic immunity to mundane weapon damage is indeed pretty powerful, but it does only apply to bludgeoning, piercing, and slashing damage. Attacks that deal different or additional kinds of damage - poison, fire, cold, etc - are still effective against lycanthropes. Lots of monsters automatically deal this sort of extra damage with their attacks, and they will remain dangerous (albeit less so) to lycanthrope characters. Normal humanoid-type enemies wielding normal weapons are going to be mostly ineffective against the characters, but that's difficult to avoid while allowing the players to remain unaltered lycanthropes.

Level 5 is one of the major milestone levels where player character power gets a dramatic jump (extra attacks, level 3 spells, etc.) anyway, so you could take the opportunity to shift focus so that the players are now getting into the realm where their challenges are more and more the sorts of fantastic creatures who do deal this kind of damage and powerful enemies of their own tier who have the magical resources and special equipment necessary to deal with them. They should still get to have a go at mundane foes from time to time, but they can be the sort of fluff encounters they're meant to handle easily and quickly rather than a serious challenge.

Use their reputation against them

It would certainly feel unreasonable if enemies who had no reason to do so suddenly started coming equipped with silvered weapons and other anti-lycanthrope measures purely because they need to be able to pose a challenge to your players. However, it would be reasonable for enemies to start using such equipment and tactics if they know they are likely to be fighting lycanthropes. If the party can't keep their nature a secret, their enemies will learn to fight them with silver and fire. Even if the characters are never seen actually transforming, there are only so many conclusions that can be drawn if they are seen to be obviously immune to mundane damage, and silvered weapons are one of the standards for overcoming such immunities.

Groups and opponents who are specifically opposed to the PCs can therefore justifiably arm themselves with silvered weapons and other anti-lycanthrope tools after the secret gets out. If the PCs are well known and active in a region, even groups who aren't specifically opposed to them might invest in having a few such weapons available, just in case these adventurers (a breed who are notorious for getting involved in other people's business) cross their path. You don't need to make every common guard or thug start carrying silver swords, but silvering is relatively inexpensive, so leaders and lieutenants at least could readily be so armed.

Retcon the damage immunity

You could houserule the nature of lycanthropy so that it only grants resistance, rather than immunity, to such mundane damage. This way your players still get a considerable advantage from becoming lycanthropes but it's not quite so game-breaking as outright immunity is. You could even flavour this as a difference between natural-born and afflicted lycanthropes, saying that natural lycanthropes have full immunity but afflicted ones only get resistance (there's already a mechanical difference between the two in that afflicted lycanthropes can be cured by Remove Curse, whereas natural ones require nothing short of a Wish).

Hopefully, you have good-natured players who would understand if you nerfed the ability a little bit in the interests of keeping your campaign grounded.