Lil' Erik:
Erik is a little more damage oriented than Klanning and I wouldn't consider him to be your tank, he is counter productive to being healthy for more damage using his W. He is better built with lots of life regen and a bit of vamp strike. If you have multiple tanks in a group, you can use the passive ability from his W on top of the First Aid kit to have a massive 7% of max health per second outside of combat to get quick revitalization.

When going for achievements, the low health ones are easily completed with Erik on Amateur 1 doing between 4-10 waves. Also the ones where you have to kill enemies who have attacked you. Since solo, everything attacks you. Another one that I believe works, I have not tested myself entirely yet, but if you are below 30% and you taunt an enemy, I believe it counts towards The Show Must Go On.

Ability order:
I would recommend maxing out his W first for quick regeneration, then I would max out his Q, since most of his scaling is from HP and Damage you really don't even need to level them up, focus Mainly on his W and R whenever available. Also remember, using your Q during his ultimate double's the bleeds damage. Which is why having a weapon that causes bleeding on Erik is a MUST!

Items:
I can't recommend too many items for Erik right now as I really don't like his design so I only used him to get the low hp achievements. He is not durable like Hardy, he is just a healthy damage dealer, and does not really fit the tank/bruiser role and his Ultimate ability I just think isn't useful at all since it doesn't last long enough nor have a noticable effect. Just get the item that applies bleed on hit, stack attack speed, and just get defensive items and spam his Q is about the only justifyable way to play him in later waves honestly.

It's unfortunate that Erik won't get much play. To me, he is basically a pseudo Dr. Mundo from League of Legends who in that concept is a great character type because you have to deal with him which is hard, or ignore him to deal with his team and let him wreak havok. In a PvE format like this, where your enemies really don't rely on focus targets, and the 10th wave boss just ignores tanking period with his random AoE spam, I just don't see him as a well designed character. His damage for basic attacks is extremely high, but the scaling of his abilities because of that is low. He is just as I said a Healthy Damage dealer. His slot is better picked by Buzzer. Buzzer's trap that taunts is just as reliable if not more reliable than Erik's taunt ability.

Hardy Klanning:
Hardy is going to be your damage reduction KING. If you max his W first you will have pretty much infinite up time which allows you to spam your R on the heroes in large groups getting those pesky healers and rocketeers out of the way quickly. With an average build you should have 75-85% damage reduction at all times. I would definitely recommend building all of Hardy's recommended and defensive items, however your first item should be the Solar Collector under the Energy Tab. Build that, then get your First Aid kit for 1000, should always be obtainable after the first wave, Then complete your B2 Power Rod, you will have near infinite energy at this point to use your abilities however you please. Mainly spamming W whenever it is up, using Q to gain group threat and E for lots of maneuverability.

Ability Order:
Start with 1 point in his E for extra mobility in dodging abilities. Then max his W asap. His Q only needs one point and I would deem it his least useful ability unless you are doing solo play. His W while active reduces his Ultimate by 1 second per attack you take, so having everything attacking you at all times for a tank makes your damage skyrocket! Note, that his ultimate can crit and heal you with vamp strike. So if you wish to incorporate that into your build, it does work nicely. I do still recommend building him pure tank with the B2 Power rod with a healer though.

As far as achievements go, the completing the 10th wave if you play your role and keep everything attacking you with a solid healer, even master wave 10 should be cake. Have done it where just myself and 1 healer steamrolled the entire thing. Most of the rest of the bruiser achievements can be done in solo play, which is possible for just about everyone in the cast. The only ones that are not possible are the revive ones.

Items:
Cerebral Overcharger
3-Rad-Icator
Hagle Breaker II
Machine That Goes Ping

Those 4 items should be your core build. Until they impliment offensive scaling defensive items, there is no better combo of items than these.

The 2 defensive items I go to most are
The Wall
Alfred's Dyna Visor

The wall slows all enemies near you and their attack speed which in turn makes you more durable and your enemies do less damage over time. The Dyna Visor I recommend since much like when you play the CC class your stuns and stuff last longer the more tech you have, the stronger enemy abilities are the longer their stuns last and in late waves having that 75% less effect of enemy effects is crucial. That on top of Hardy's W giving you reduction means that any debuffs on you don't even last a full second if at all.

Have had people recommending that I give mention to the Nemesis Crux. This item does have a fantastic active ability, I just cannot however justify it as a good item. The 10% burst of damage will not change the outcome of a fight, nor will it have much effect in keeping your team alive or making you more durable. If the effect did 10% damage and healed your entire party, then possibly it could be ok, but if you are playing Hardy, and you are solo, then maybe you can impliment it into your build.

Other potential items:
Attractive Magnet Boots
The Windmiller

The boots will allow you to pull enemies in for quick aoe killing in groups, and also during slower waves allow you to go and clear side camps really quickly without being reliant on heals. The Windmiller is just for bonus dps if you don't want to build tanky but with the change to his W and tech not giving you extra armor I wouldn't recommend this anymore unless you're playing lower difficulties.

I do not get my B2 Power Rod anymore but for new players, until you get better at energy management, keep using that as your starting item as it will allow you to use your abilities indefinitely.

I will continue to update this guide mostly for Hardy and I feel bad that Erik doesn't get much play but I just don't like his design.